diff --git a/error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/SpecifyLocale.java b/error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/SpecifyLocale.java
new file mode 100644
index 00000000000..ad2e96d9f0c
--- /dev/null
+++ b/error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/SpecifyLocale.java
@@ -0,0 +1,63 @@
+package tech.picnic.errorprone.bugpatterns;
+
+import static com.google.errorprone.BugPattern.LinkType.CUSTOM;
+import static com.google.errorprone.BugPattern.SeverityLevel.WARNING;
+import static com.google.errorprone.BugPattern.StandardTags.LIKELY_ERROR;
+import static com.google.errorprone.matchers.method.MethodMatchers.instanceMethod;
+import static tech.picnic.errorprone.bugpatterns.util.Documentation.BUG_PATTERNS_BASE_URL;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.bugpatterns.BugChecker.MethodInvocationTreeMatcher;
+import com.google.errorprone.fixes.Fix;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+import tech.picnic.errorprone.bugpatterns.util.SourceCode;
+
+/**
+ * A {@link BugChecker} that flags {@link String#toLowerCase()} or {@link String#toUpperCase()}
+ * which do not specify a Locale.
+ */
+@AutoService(BugChecker.class)
+@BugPattern(
+    summary =
+        "Specify `Locale.ROOT` or `Locale.getDefault()` when calling `String#to{Lower,Upper}Case` without a specific Locale",
+    link = BUG_PATTERNS_BASE_URL + "SpecifyLocale",
+    linkType = CUSTOM,
+    severity = WARNING,
+    tags = LIKELY_ERROR)
+public final class SpecifyLocale extends BugChecker implements MethodInvocationTreeMatcher {
+  private static final long serialVersionUID = 1L;
+  private static final Matcher<ExpressionTree> STRING_TO_UPPER_OR_LOWER_CASE =
+      instanceMethod()
+          .onExactClass(String.class.getName())
+          .namedAnyOf("toLowerCase", "toUpperCase");
+
+  /** Instantiates a new {@link SpecifyLocale} instance. */
+  public SpecifyLocale() {}
+
+  @Override
+  public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState state) {
+    if (STRING_TO_UPPER_OR_LOWER_CASE.matches(tree, state) && tree.getArguments().isEmpty()) {
+      return buildDescription(tree)
+          .addFix(buildFix("Locale.ROOT", tree, state))
+          .addFix(buildFix("Locale.getDefault()", tree, state))
+          .build();
+    }
+    return Description.NO_MATCH;
+  }
+
+  private static Fix buildFix(
+      String localeToSpecify, MethodInvocationTree tree, VisitorState state) {
+    return SuggestedFix.builder()
+        .replace(
+            tree, SourceCode.treeToString(tree, state).replace("()", "(" + localeToSpecify + ")"))
+        .addImport("java.util.Locale")
+        .build();
+  }
+}
diff --git a/error-prone-contrib/src/test/java/tech/picnic/errorprone/bugpatterns/SpecifyLocaleTest.java b/error-prone-contrib/src/test/java/tech/picnic/errorprone/bugpatterns/SpecifyLocaleTest.java
new file mode 100644
index 00000000000..d611a62cc64
--- /dev/null
+++ b/error-prone-contrib/src/test/java/tech/picnic/errorprone/bugpatterns/SpecifyLocaleTest.java
@@ -0,0 +1,95 @@
+package tech.picnic.errorprone.bugpatterns;
+
+import static com.google.errorprone.BugCheckerRefactoringTestHelper.newInstance;
+
+import com.google.errorprone.BugCheckerRefactoringTestHelper;
+import com.google.errorprone.BugCheckerRefactoringTestHelper.FixChoosers;
+import com.google.errorprone.BugCheckerRefactoringTestHelper.TestMode;
+import com.google.errorprone.CompilationTestHelper;
+import org.junit.jupiter.api.Test;
+
+final class SpecifyLocaleTest {
+  private final CompilationTestHelper compilationTestHelper =
+      CompilationTestHelper.newInstance(SpecifyLocale.class, getClass());
+  private final BugCheckerRefactoringTestHelper refactoringTestHelper =
+      newInstance(SpecifyLocale.class, getClass());
+
+  @Test
+  void identification() {
+    compilationTestHelper
+        .addSourceLines(
+            "A.java",
+            "import java.util.Locale;",
+            "",
+            "class A {",
+            "  void m() {",
+            "    // BUG: Diagnostic contains:",
+            "    \"a\".toUpperCase();",
+            "",
+            "    // BUG: Diagnostic contains:",
+            "    \"b\".toLowerCase();",
+            "",
+            "    String c = \"c\";",
+            "    // BUG: Diagnostic contains:",
+            "    c.toUpperCase();",
+            "",
+            "    String d = \"d\";",
+            "    // BUG: Diagnostic contains:",
+            "    d.toLowerCase();",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  void replacementFirstSuggestedFix() {
+    refactoringTestHelper
+        .setFixChooser(FixChoosers.FIRST)
+        .addInputLines(
+            "A.java",
+            "",
+            "class A {",
+            "  void m() {",
+            "    \"a\".toUpperCase();",
+            "    \"b\".toLowerCase();",
+            "  }",
+            "}")
+        .addOutputLines(
+            "A.java",
+            "import java.util.Locale;",
+            "",
+            "class A {",
+            "  void m() {",
+            "    \"a\".toUpperCase(Locale.ROOT);",
+            "    \"b\".toLowerCase(Locale.ROOT);",
+            "  }",
+            "}")
+        .doTest(TestMode.TEXT_MATCH);
+  }
+
+  @Test
+  void replacementSecondSuggestedFix() {
+    refactoringTestHelper
+        .setFixChooser(FixChoosers.SECOND)
+        .addInputLines(
+            "A.java",
+            "",
+            "class A {",
+            "  void m() {",
+            "    \"a\".toUpperCase();",
+            "    \"b\".toLowerCase();",
+            "  }",
+            "}")
+        .addOutputLines(
+            "A.java",
+            "import java.util.Locale;",
+            "",
+            "class A {",
+            "  void m() {",
+            "    \"a\".toUpperCase(Locale.getDefault());",
+            "    \"b\".toLowerCase(Locale.getDefault());",
+            "  }",
+            "}")
+        .doTest(TestMode.TEXT_MATCH);
+  }
+}