diff --git a/WordPress/Sniffs/Functions/ReturnTypeSniff.php b/WordPress/Sniffs/Functions/ReturnTypeSniff.php
new file mode 100644
index 0000000000..4f99bbae56
--- /dev/null
+++ b/WordPress/Sniffs/Functions/ReturnTypeSniff.php
@@ -0,0 +1,140 @@
+<?php
+/**
+ * WordPress Coding Standard.
+ *
+ * @package WPCS\WordPressCodingStandards
+ * @link    https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards
+ * @license https://opensource.org/licenses/MIT MIT
+ */
+
+namespace WordPress\Sniffs\Functions;
+
+use PHP_CodeSniffer_Tokens as Tokens;
+use WordPress\Sniff;
+
+/**
+ * Enforces formatting of return types.
+ *
+ * @package WPCS\WordPressCodingStandards
+ *
+ * @since   0.14.0
+ *
+ * @link    https://github.com/zendframework/zend-coding-standard/blob/52b435c714879609262aa36b004c4075cd967acc/src/ZendCodingStandard/Sniffs/Formatting/ReturnTypeSniff.php
+ */
+class ReturnTypeSniff extends Sniff {
+	/**
+	 * Simple return types.
+	 *
+	 * @since 0.14.0
+	 *
+	 * @var string[]
+	 */
+	private $simple_return_types = array(
+		'void',
+		'int',
+		'float',
+		'double',
+		'string',
+		'array',
+		'iterable',
+		'callable',
+		'parent',
+		'self',
+		'bool',
+	);
+
+	/**
+	 * Returns an array of tokens this test wants to listen for.
+	 *
+	 * @return array
+	 */
+	public function register() {
+		return array(
+			T_RETURN_TYPE,
+		);
+	}
+
+	/**
+	 * Processes this test, when one of its tokens is encountered.
+	 *
+	 * @param int $stackPtr The position of the current token in the stack.
+	 */
+	public function process_token( $stackPtr ) {
+		$colon = (int) $this->phpcsFile->findPrevious( T_COLON, $stackPtr - 1 );
+
+		// Space before colon disallowed.
+		if ( T_CLOSE_PARENTHESIS !== $this->tokens[ $colon - 1 ]['code'] ) {
+			$error = 'There must be no space before colon before a return type.';
+			$fix   = $this->phpcsFile->addFixableError( $error, $colon - 1, 'SpaceBeforeColon' );
+
+			if ( true === $fix ) {
+				$this->phpcsFile->fixer->beginChangeset();
+				$token = $colon - 1;
+				do {
+					$this->phpcsFile->fixer->replaceToken( $token, '' );
+					-- $token;
+				} while ( T_CLOSE_PARENTHESIS !== $this->tokens[ $token ]['code'] );
+				$this->phpcsFile->fixer->endChangeset();
+			}
+		}
+
+		// Only one space after colon.
+		if ( T_WHITESPACE !== $this->tokens[ $colon + 1 ]['code'] ) {
+			$error = 'There must be a space after colon and before return type declaration.';
+			$fix   = $this->phpcsFile->addFixableError( $error, $colon, 'NoSpaceAfterColon' );
+			if ( true === $fix ) {
+				$this->phpcsFile->fixer->addContent( $colon, ' ' );
+			}
+		} elseif ( ' ' !== $this->tokens[ $colon + 1 ]['content'] ) {
+			$error = 'There must be exactly one space after colon and before return type declaration.';
+			$fix   = $this->phpcsFile->addFixableError( $error, $colon + 1, 'TooManySpacesAfterColon' );
+			if ( true === $fix ) {
+				$this->phpcsFile->fixer->replaceToken( $colon + 1, ' ' );
+			}
+		}
+
+		$nullable = $this->phpcsFile->findNext( T_NULLABLE, $colon + 1, $stackPtr );
+		if ( $nullable ) {
+			// Check if there is space after nullable operator.
+			if ( T_WHITESPACE === $this->tokens[ $nullable + 1 ]['code'] ) {
+				$error = 'Space is not not allowed after nullable operator.';
+				$fix   = $this->phpcsFile->addFixableError( $error, $nullable + 1, 'SpaceAfterNullable' );
+				if ( $fix ) {
+					$this->phpcsFile->fixer->replaceToken( $nullable + 1, '' );
+				}
+			}
+		}
+
+		$first   = $this->phpcsFile->findNext( Tokens::$emptyTokens, ( $nullable ?: $colon ) + 1, null, true );
+		$end     = $this->phpcsFile->findNext( [ T_SEMICOLON, T_OPEN_CURLY_BRACKET ], $stackPtr + 1 );
+		$last    = $this->phpcsFile->findPrevious( Tokens::$emptyTokens, $end - 1, null, true );
+		$invalid = $this->phpcsFile->findNext( [ T_STRING, T_NS_SEPARATOR, T_RETURN_TYPE ], $first, $last + 1, true );
+		if ( $invalid ) {
+			$error = 'Return type declaration contains invalid token %s';
+			$data  = array( $this->tokens[ $invalid ]['type'] );
+			$fix   = $this->phpcsFile->addFixableError( $error, $invalid, 'InvalidToken', $data );
+			if ( true === $fix ) {
+				$this->phpcsFile->fixer->replaceToken( $invalid, '' );
+			}
+
+			return;
+		}
+
+		$returnType = trim( $this->phpcsFile->getTokensAsString( $first, $last - $first + 1 ) );
+
+		if ( $first === $last
+			&& in_array( strtolower( $returnType ), $this->simple_return_types, true )
+			&& ! in_array( $returnType, $this->simple_return_types, true )
+		) {
+			$error = 'Simple return type must be lowercase. Found "%s", expected "%s"';
+			$data  = array(
+				$returnType,
+				strtolower( $returnType ),
+			);
+			$fix   = $this->phpcsFile->addFixableError( $error, $first, 'LowerCaseSimpleType', $data );
+			if ( true === $fix ) {
+				$this->phpcsFile->fixer->replaceToken( $stackPtr, strtolower( $returnType ) );
+			}
+		}
+	}
+}
diff --git a/WordPress/Tests/Functions/ReturnTypeUnitTest.inc b/WordPress/Tests/Functions/ReturnTypeUnitTest.inc
new file mode 100644
index 0000000000..7f9867ea18
--- /dev/null
+++ b/WordPress/Tests/Functions/ReturnTypeUnitTest.inc
@@ -0,0 +1,84 @@
+<?php
+
+function fooa(): array {} // Good.
+function foob() : array {} // Bad.
+function fooc()     : array {} // Bad.
+function food() // Bad.
+: array {}
+function fooe():array {} // Bad.
+function foof():    array {} // Bad.
+function foog():
+array {}
+
+// Don't touch case statements here.
+$v = 1;
+switch ($v) {
+	case 1:
+		$x = $f1(-1 * $v);
+		break;
+	case (2) :
+		$x = $f2(-1 * $v, $v);
+		break;
+	default:
+		$x = $f1($v) + $f2(-1 * $v, $v);
+		break;
+}
+
+class ReturnType
+{
+	public function method() :array // Bad.
+	{
+		return [];
+	}
+
+	private function priv(
+		$a,
+		$b
+	)
+	: int { // Bad.
+		return $a * $b;
+	}
+}
+
+$c = new class() {
+	public function method() :
+	float {
+		return mt_rand();
+	}
+};
+
+abstract class AbsCla
+{
+	abstract public function x() :bool; // Bad.
+}
+
+interface MyInterface
+{ // All below are bad.
+	public function a():vOid;
+	public function b() : Int;
+	public function c() :?int;
+	public function d() :Float;
+	public function e() :? float;
+	public function f():Double;
+	public function g():?double;
+	public function h():Array;
+	public function i() : ?array;
+	public function j():String;
+	public function k():?string;
+	public function l():Parent;
+	public function m():?parent;
+	public function n():Callable;
+	public function o():?callable;
+	public function p():Bool;
+	public function q():?bool;
+	public function r():Self;
+	public function s():?self;
+	public function t():Iterable;
+	public function u():?iterable;
+
+	public function v($a) :  \DateTime;
+	public function w():?\DateTime;
+
+	public function y($a, $b, $c) : \My\TestClass;
+	public function z():? \ReturnType \MyType;
+}
diff --git a/WordPress/Tests/Functions/ReturnTypeUnitTest.inc.fixed b/WordPress/Tests/Functions/ReturnTypeUnitTest.inc.fixed
new file mode 100644
index 0000000000..a19d6c4842
--- /dev/null
+++ b/WordPress/Tests/Functions/ReturnTypeUnitTest.inc.fixed
@@ -0,0 +1,80 @@
+<?php
+
+function fooa(): array {} // Good.
+function foob(): array {} // Bad.
+function fooc(): array {} // Bad.
+function food(): array {}
+function fooe(): array {} // Bad.
+function foof(): array {} // Bad.
+function foog(): array {}
+
+// Don't touch case statements here.
+$v = 1;
+switch ($v) {
+	case 1:
+		$x = $f1(-1 * $v);
+		break;
+	case (2) :
+		$x = $f2(-1 * $v, $v);
+		break;
+	default:
+		$x = $f1($v) + $f2(-1 * $v, $v);
+		break;
+}
+
+class ReturnType
+{
+	public function method(): array // Bad.
+	{
+		return [];
+	}
+
+	private function priv(
+		$a,
+		$b
+	): int { // Bad.
+		return $a * $b;
+	}
+}
+
+$c = new class() {
+	public function method(): float {
+		return mt_rand();
+	}
+};
+
+abstract class AbsCla
+{
+	abstract public function x(): bool; // Bad.
+}
+
+interface MyInterface
+{ // All below are bad.
+	public function a(): void;
+	public function b(): int;
+	public function c(): ?int;
+	public function d(): float;
+	public function e(): ?float;
+	public function f(): double;
+	public function g(): ?double;
+	public function h(): array;
+	public function i(): ?array;
+	public function j(): string;
+	public function k(): ?string;
+	public function l(): parent;
+	public function m(): ?parent;
+	public function n(): callable;
+	public function o(): ?callable;
+	public function p(): bool;
+	public function q(): ?bool;
+	public function r(): self;
+	public function s(): ?self;
+	public function t(): iterable;
+	public function u(): ?iterable;
+
+	public function v($a): \DateTime;
+	public function w(): ?\DateTime;
+
+	public function y($a, $b, $c): \My\TestClass;
+	public function z(): ?\ReturnType\MyType;
+}
diff --git a/WordPress/Tests/Functions/ReturnTypeUnitTest.php b/WordPress/Tests/Functions/ReturnTypeUnitTest.php
new file mode 100644
index 0000000000..c11c1acc82
--- /dev/null
+++ b/WordPress/Tests/Functions/ReturnTypeUnitTest.php
@@ -0,0 +1,78 @@
+<?php
+/**
+ * Unit test class for WordPress Coding Standard.
+ *
+ * @package WPCS\WordPressCodingStandards
+ * @link    https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards
+ * @license https://opensource.org/licenses/MIT MIT
+ */
+
+namespace WordPress\Tests\Functions;
+
+use PHP_CodeSniffer\Tests\Standards\AbstractSniffUnitTest;
+
+/**
+ * Unit test class for the ReturnType sniff.
+ *
+ * @package WPCS\WordPressCodingStandards
+ *
+ * @since   0.14.0
+ */
+class ReturnTypeUnitTest extends AbstractSniffUnitTest {
+
+	/**
+	 * Returns the lines where errors should occur.
+	 *
+	 * @return array <int line number> => <int number of errors>
+	 */
+	public function getErrorList() {
+		return array(
+			4 => 1,
+			5 => 1,
+			6 => 1,
+			8 => 1,
+			9 => 1,
+			10 => 1,
+			29 => 2,
+			38 => 1,
+			44 => 2,
+			52 => 2,
+			57 => 2,
+			58 => 2,
+			59 => 2,
+			60 => 3,
+			61 => 3,
+			62 => 2,
+			63 => 1,
+			64 => 2,
+			65 => 1,
+			66 => 2,
+			67 => 1,
+			68 => 2,
+			69 => 1,
+			70 => 2,
+			71 => 1,
+			72 => 2,
+			73 => 1,
+			74 => 2,
+			75 => 1,
+			76 => 2,
+			77 => 1,
+			79 => 2,
+			80 => 1,
+			82 => 1,
+			83 => 3,
+		);
+	}
+
+	/**
+	 * Returns the lines where warnings should occur.
+	 *
+	 * @return array <int line number> => <int number of warnings>
+	 */
+	public function getWarningList() {
+		return array();
+
+	}
+
+}