diff --git a/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/XMLSchemaErrorCode.java b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/XMLSchemaErrorCode.java
index ebc900d46c..51be2d5502 100644
--- a/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/XMLSchemaErrorCode.java
+++ b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/XMLSchemaErrorCode.java
@@ -24,6 +24,8 @@
 import org.eclipse.lemminx.dom.DOMNode;
 import org.eclipse.lemminx.dom.NoNamespaceSchemaLocation;
 import org.eclipse.lemminx.dom.SchemaLocation;
+import org.eclipse.lemminx.extensions.contentmodel.participants.codeactions.TargetNamespace_1CodeAction;
+import org.eclipse.lemminx.extensions.contentmodel.participants.codeactions.TargetNamespace_2CodeAction;
 import org.eclipse.lemminx.extensions.contentmodel.participants.codeactions.cvc_attribute_3CodeAction;
 import org.eclipse.lemminx.extensions.contentmodel.participants.codeactions.cvc_complex_type_2_1CodeAction;
 import org.eclipse.lemminx.extensions.contentmodel.participants.codeactions.cvc_complex_type_2_3CodeAction;
@@ -74,6 +76,7 @@ public enum XMLSchemaErrorCode implements IXMLErrorCode {
 	cvc_maxInclusive_valid("cvc-maxInclusive-valid"), // https://wiki.xmldation.com/Support/validator/cvc-maxinclusive-valid
 	cvc_minExclusive_valid("cvc-minExclusive-valid"), // https://wiki.xmldation.com/Support/validator/cvc-minexclusive-valid
 	cvc_minInclusive_valid("cvc-minInclusive-valid"), // https://wiki.xmldation.com/Support/validator/cvc-mininclusive-valid
+	TargetNamespace_1("TargetNamespace.1"), //
 	TargetNamespace_2("TargetNamespace.2"), SchemaLocation("SchemaLocation"), schema_reference_4("schema_reference.4"), //
 	src_element_3("src-element.3");
 
@@ -138,7 +141,6 @@ public static Range toLSPRange(XMLLocator location, XMLSchemaErrorCode code, Obj
 		case cvc_elt_1_a:
 		case cvc_complex_type_4:
 		case src_element_3:
-		case TargetNamespace_2:
 			return XMLPositionUtility.selectStartTagName(offset, document);
 		case cvc_complex_type_3_2_2: {
 			String attrName = getString(arguments[1]);
@@ -245,6 +247,10 @@ public static Range toLSPRange(XMLLocator location, XMLSchemaErrorCode code, Obj
 		}
 		case cvc_type_3_1_2:
 			return XMLPositionUtility.selectStartTagName(offset, document);
+		case TargetNamespace_1:
+			return XMLPositionUtility.selectRootAttributeValue("xmlns", document);
+		case TargetNamespace_2:
+			return XMLPositionUtility.selectRootStartTag(document);
 		default:
 		}
 		return null;
@@ -260,5 +266,7 @@ public static void registerCodeActionParticipants(Map<String, ICodeActionPartici
 		codeActions.put(cvc_complex_type_3_2_2.getCode(), new cvc_complex_type_3_2_2CodeAction());
 		codeActions.put(cvc_enumeration_valid.getCode(), new cvc_enumeration_validCodeAction());
 		codeActions.put(cvc_complex_type_2_1.getCode(), new cvc_complex_type_2_1CodeAction());
+		codeActions.put(TargetNamespace_1.getCode(), new TargetNamespace_1CodeAction());
+		codeActions.put(TargetNamespace_2.getCode(), new TargetNamespace_2CodeAction());
 	}
 }
diff --git a/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/codeactions/TargetNamespace_1CodeAction.java b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/codeactions/TargetNamespace_1CodeAction.java
new file mode 100644
index 0000000000..da9c8964c8
--- /dev/null
+++ b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/codeactions/TargetNamespace_1CodeAction.java
@@ -0,0 +1,69 @@
+/**
+ * Copyright (c) 2020 Red Hat Inc. and others.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v2.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v20.html
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ * Red Hat Inc. - initial API and implementation
+ */
+package org.eclipse.lemminx.extensions.contentmodel.participants.codeactions;
+
+import java.util.List;
+import java.util.regex.Pattern;
+import java.util.regex.Matcher;
+
+import org.eclipse.lemminx.commons.CodeActionFactory;
+import org.eclipse.lemminx.dom.DOMDocument;
+import org.eclipse.lemminx.services.extensions.ICodeActionParticipant;
+import org.eclipse.lemminx.services.extensions.IComponentProvider;
+import org.eclipse.lemminx.settings.SharedSettings;
+import org.eclipse.lemminx.utils.StringUtils;
+import org.eclipse.lsp4j.CodeAction;
+import org.eclipse.lsp4j.Diagnostic;
+import org.eclipse.lsp4j.Range;
+
+/**
+ * CodeAction to replace an incorrect namespace in an .xml document.
+ * 
+ * Changes the value of the xmlns attribute of the root element of the .xml
+ * document to the declared namespace of the referenced .xsd document.
+ */
+public class TargetNamespace_1CodeAction implements ICodeActionParticipant {
+
+	private static final Pattern NAMESPACE_EXTRACTOR = Pattern.compile("'([^']+)'\\.");
+
+	@Override
+	public void doCodeAction(Diagnostic diagnostic, Range range, DOMDocument document, List<CodeAction> codeActions,
+			SharedSettings sharedSettings, IComponentProvider componentProvider) {
+
+		String namespace = extractNamespace(diagnostic.getMessage());
+		if (StringUtils.isEmpty(namespace)) {
+			return;
+		}
+		String quote = sharedSettings.getPreferences().getQuotationAsString();
+		// @formatter:off
+		CodeAction replaceNamespace = CodeActionFactory.replace(
+				"Replace with '" + namespace + "'",
+				diagnostic.getRange(),
+				quote + namespace + quote,
+				document.getTextDocument(), diagnostic);
+		// @formatter:on
+		codeActions.add(replaceNamespace);
+	}
+
+	private static String extractNamespace(String diagnosticMessage) {
+		// The error message has this form:
+		// TargetNamespace.1: Expecting namespace 'NaN', but the target namespace of the
+		// schema document is 'http://two-letter-name'.
+		Matcher nsMatcher = NAMESPACE_EXTRACTOR.matcher(diagnosticMessage);
+		if (nsMatcher.find()) {
+			return nsMatcher.group(1);
+		}
+		return null;
+	}
+
+}
diff --git a/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/codeactions/TargetNamespace_2CodeAction.java b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/codeactions/TargetNamespace_2CodeAction.java
new file mode 100644
index 0000000000..102286a18b
--- /dev/null
+++ b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/codeactions/TargetNamespace_2CodeAction.java
@@ -0,0 +1,79 @@
+/**
+ * Copyright (c) 2020 Red Hat Inc. and others.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v2.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v20.html
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ * Red Hat Inc. - initial API and implementation
+ */
+package org.eclipse.lemminx.extensions.contentmodel.participants.codeactions;
+
+import java.util.List;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.eclipse.lemminx.commons.CodeActionFactory;
+import org.eclipse.lemminx.dom.DOMDocument;
+import org.eclipse.lemminx.dom.DOMNode;
+import org.eclipse.lemminx.services.extensions.ICodeActionParticipant;
+import org.eclipse.lemminx.services.extensions.IComponentProvider;
+import org.eclipse.lemminx.settings.SharedSettings;
+import org.eclipse.lemminx.utils.StringUtils;
+import org.eclipse.lemminx.utils.XMLPositionUtility;
+import org.eclipse.lsp4j.CodeAction;
+import org.eclipse.lsp4j.Diagnostic;
+import org.eclipse.lsp4j.Position;
+import org.eclipse.lsp4j.Range;
+
+/**
+ * Code action to add the missing xmlns declaration to the root element in an
+ * .xml document.
+ * 
+ * Finds the namespace of the referenced .xsd. Adds the xmlns attribute to the
+ * root of .xml, and sets its value to the .xsd namespace.
+ */
+public class TargetNamespace_2CodeAction implements ICodeActionParticipant {
+
+	private static final Pattern NAMESPACE_EXTRACTOR = Pattern.compile("'([^']+)'\\.");
+
+	@Override
+	public void doCodeAction(Diagnostic diagnostic, Range range, DOMDocument document, List<CodeAction> codeActions,
+			SharedSettings sharedSettings, IComponentProvider componentProvider) {
+
+		String namespace = extractNamespace(diagnostic.getMessage());
+		if (StringUtils.isEmpty(namespace)) {
+			return;
+		}
+		DOMNode root = document.getDocumentElement();
+		if (root == null) {
+			return;
+		}
+		Position tagEnd = XMLPositionUtility.selectStartTagName(root).getEnd();
+		String quote = sharedSettings.getPreferences().getQuotationAsString();
+		// @formatter:off
+		CodeAction addNamespaceDecl = CodeActionFactory.insert(
+				"Declare '" + namespace + "' as the namespace",
+				tagEnd,
+				" xmlns=" + quote + namespace + quote,
+				document.getTextDocument(),
+				diagnostic);
+		// @formatter:on
+		codeActions.add(addNamespaceDecl);
+	}
+
+	private static String extractNamespace(String diagnosticMessage) {
+		// The error message has this form:
+		// TargetNamespace.2: Expecting no namespace, but the schema document has a
+		// target namespace of 'http://docbook.org/ns/docbook'.
+		Matcher nsMatcher = NAMESPACE_EXTRACTOR.matcher(diagnosticMessage);
+		if (nsMatcher.find()) {
+			return nsMatcher.group(1);
+		}
+		return null;
+	}
+
+}
diff --git a/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/utils/XMLPositionUtility.java b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/utils/XMLPositionUtility.java
index 5f1749faf7..8f9011b2e5 100644
--- a/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/utils/XMLPositionUtility.java
+++ b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/utils/XMLPositionUtility.java
@@ -325,6 +325,34 @@ public static Range selectRootStartTag(DOMDocument document) {
 		return selectStartTagName(root);
 	}
 
+	/**
+	 * Finds the root element of the given document and returns the
+	 * attribute value <code>Range</code> for the attribute
+	 * <code>attrName</code>.
+	 * 
+	 * If <code>attrName</code> is not declared then null is returned.
+	 * 
+	 * @param attrName The name of the attribute to find the range of the value for
+	 * @param document The document to use the root element of
+	 * @return The range in <code>document</code> where the declared value of
+	 *         attribute <code>attrName</code> resides (including quotations),
+	 *         or null if the attriubte is not declared.
+	 */
+	public static Range selectRootAttributeValue(String attrName, DOMDocument document) {
+		DOMNode root = document.getDocumentElement();
+		if (root == null) {
+			root = document.getChild(0);
+		}
+		if (root == null) {
+			return null;
+		}
+		DOMAttr attr = root.getAttributeNode(attrName);
+		if (attr == null) {
+			return null;
+		}
+		return selectAttributeValue(attr);
+	}
+
 	public static Range selectStartTagName(int offset, DOMDocument document) {
 		DOMNode element = document.findNodeAt(offset);
 		if (element != null) {
diff --git a/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLSchemaDiagnosticsTest.java b/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLSchemaDiagnosticsTest.java
index e6d811ad1b..b2fa045655 100644
--- a/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLSchemaDiagnosticsTest.java
+++ b/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLSchemaDiagnosticsTest.java
@@ -20,6 +20,9 @@
 import org.eclipse.lemminx.XMLAssert;
 import org.eclipse.lemminx.extensions.contentmodel.participants.XMLSchemaErrorCode;
 import org.eclipse.lemminx.extensions.contentmodel.settings.ContentModelSettings;
+import org.eclipse.lemminx.settings.EnforceQuoteStyle;
+import org.eclipse.lemminx.settings.QuoteStyle;
+import org.eclipse.lemminx.settings.SharedSettings;
 import org.eclipse.lsp4j.Diagnostic;
 import org.junit.jupiter.api.Test;
 
@@ -577,6 +580,69 @@ public void cvc_complex_type_2_2_withText() throws Exception {
 		testDiagnosticsFor(xml, diagnosticBob, diagnostic_cvc_2_2);
 	}
 
+	@Test
+	public void testTargetNamespace_1Normal() throws Exception {
+		String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n" + //
+				"<?xml-model href=\"src/test/resources/xsd/two-letter-name.xsd\"?>\n" + //
+				"<two-letter-name xmlns=\"BAD_NS\">Io</two-letter-name>";
+		Diagnostic targetNamespace = d(2, 23, 2, 31, XMLSchemaErrorCode.TargetNamespace_1, "TargetNamespace.1: Expecting namespace 'BAD_NS', but the target namespace of the schema document is 'http://two-letter-name'.");
+		testDiagnosticsFor(xml,
+				targetNamespace,
+				d(2, 1, 2, 16, XMLSchemaErrorCode.cvc_elt_1_a, "cvc-elt.1.a: Cannot find the declaration of element 'two-letter-name'.")
+		);
+		testCodeActionsFor(xml, targetNamespace, ca(targetNamespace, te(2, 23, 2, 31, "\"http://two-letter-name\"")));
+	}
+
+	@Test
+	public void testTargetNamespace_1ShortNS() throws Exception {
+		String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n" + //
+				"<?xml-model href=\"src/test/resources/xsd/two-letter-name.xsd\"?>\n" + //
+				"<two-letter-name xmlns=\"_\">Io</two-letter-name>";
+		Diagnostic targetNamespace = d(2, 23, 2, 26, XMLSchemaErrorCode.TargetNamespace_1, "TargetNamespace.1: Expecting namespace '_', but the target namespace of the schema document is 'http://two-letter-name'.");
+		testDiagnosticsFor(xml,
+				targetNamespace,
+				d(2, 1, 2, 16, XMLSchemaErrorCode.cvc_elt_1_a, "cvc-elt.1.a: Cannot find the declaration of element 'two-letter-name'.")
+		);
+		testCodeActionsFor(xml, targetNamespace, ca(targetNamespace, te(2, 23, 2, 26, "\"http://two-letter-name\"")));
+	}
+	
+	@Test
+	public void testTargetNamespace_1SingleQuotes() throws Exception {
+		String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n" + //
+				"<?xml-model href=\"src/test/resources/xsd/two-letter-name.xsd\"?>\n" + //
+				"<two-letter-name xmlns=\"_\">Io</two-letter-name>";
+		SharedSettings settings = new SharedSettings();
+		settings.getFormattingSettings().setEnforceQuoteStyle(EnforceQuoteStyle.preferred);
+		settings.getPreferences().setQuoteStyle(QuoteStyle.singleQuotes);
+		Diagnostic targetNamespace = d(2, 23, 2, 26, XMLSchemaErrorCode.TargetNamespace_1, "TargetNamespace.1: Expecting namespace '_', but the target namespace of the schema document is 'http://two-letter-name'.");
+		testCodeActionsFor(xml, targetNamespace, settings, ca(targetNamespace, te(2, 23, 2, 26, "'http://two-letter-name'")));
+	}
+
+	@Test
+	public void testTargetNamespace_2() throws Exception {
+		String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n" + //
+				"<?xml-model href=\"src/test/resources/xsd/two-letter-name.xsd\"?>\n" + //
+				"<two-letter-name>Io</two-letter-name>";
+		Diagnostic targetNamespace = d(2, 1, 2, 16, XMLSchemaErrorCode.TargetNamespace_2, "TargetNamespace.2: Expecting no namespace, but the schema document has a target namespace of 'http://two-letter-name'.");
+		testDiagnosticsFor(xml,
+				targetNamespace,
+				d(2, 1, 2, 16, XMLSchemaErrorCode.cvc_elt_1_a, "cvc-elt.1.a: Cannot find the declaration of element 'two-letter-name'.")
+		);
+		testCodeActionsFor(xml, targetNamespace, ca(targetNamespace, te(2, 16, 2, 16, " xmlns=\"http://two-letter-name\"")));
+	}
+
+	@Test
+	public void testTargetNamespace_2SingleQuotes() throws Exception {
+		String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n" + //
+				"<?xml-model href=\"src/test/resources/xsd/two-letter-name.xsd\"?>\n" + //
+				"<two-letter-name>Io</two-letter-name>";
+		SharedSettings settings = new SharedSettings();
+		settings.getFormattingSettings().setEnforceQuoteStyle(EnforceQuoteStyle.preferred);
+		settings.getPreferences().setQuoteStyle(QuoteStyle.singleQuotes);
+		Diagnostic targetNamespace = d(2, 1, 2, 16, XMLSchemaErrorCode.TargetNamespace_2, "TargetNamespace.2: Expecting no namespace, but the schema document has a target namespace of 'http://two-letter-name'.");
+		testCodeActionsFor(xml, targetNamespace, settings, ca(targetNamespace, te(2, 16, 2, 16, " xmlns='http://two-letter-name'")));
+	}
+
 	private static void testDiagnosticsFor(String xml, Diagnostic... expected) {
 		XMLAssert.testDiagnosticsFor(xml, "src/test/resources/catalogs/catalog.xml", expected);
 	}
diff --git a/org.eclipse.lemminx/src/test/resources/xsd/two-letter-name.xsd b/org.eclipse.lemminx/src/test/resources/xsd/two-letter-name.xsd
new file mode 100644
index 0000000000..83ede5f3a1
--- /dev/null
+++ b/org.eclipse.lemminx/src/test/resources/xsd/two-letter-name.xsd
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://two-letter-name">
+    <xs:element name="two-letter-name">
+        <xs:simpleType>
+            <xs:restriction base="xs:token">
+                <xs:pattern value="[A-Z][a-z]"></xs:pattern>
+            </xs:restriction>
+        </xs:simpleType>
+    </xs:element>
+</xs:schema>
\ No newline at end of file