diff --git a/inc/types/class-authorization-code.php b/inc/types/class-authorization-code.php
index dae2470..62a9679 100644
--- a/inc/types/class-authorization-code.php
+++ b/inc/types/class-authorization-code.php
@@ -61,7 +61,7 @@ protected function handle_authorization_submission( $submit, Client $client, $da
 				);
 		}
 
-		if ( ! empty( $data['state'] ) ) {
+		if ( isset( $data['state'] ) ) {
 			$redirect_args['state'] = $data['state'];
 		}
 
diff --git a/inc/types/class-base.php b/inc/types/class-base.php
index 3c675b0..a35cf13 100644
--- a/inc/types/class-base.php
+++ b/inc/types/class-base.php
@@ -44,7 +44,8 @@ public function handle_authorisation() {
 		$client_id    = sanitize_text_field( wp_unslash( $_GET['client_id'] ) );
 		$redirect_uri = isset( $_GET['redirect_uri'] ) ? sanitize_text_field( wp_unslash( $_GET['redirect_uri'] ) ) : null;
 		$scope        = isset( $_GET['scope'] ) ? sanitize_text_field( wp_unslash( $_GET['scope'] ) ) : null;
-		$state        = isset( $_GET['state'] ) ? sanitize_text_field( wp_unslash( $_GET['state'] ) ) : null;
+		// State is opaque to the server and must be returned to the client unchanged.
+		$state = isset( $_GET['state'] ) && is_string( $_GET['state'] ) ? wp_unslash( $_GET['state'] ) : null; // phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized
 
 		$client = Client::get_by_id( $client_id );
 		if ( empty( $client ) ) {
diff --git a/inc/types/class-implicit.php b/inc/types/class-implicit.php
index 35151f5..c9e9162 100644
--- a/inc/types/class-implicit.php
+++ b/inc/types/class-implicit.php
@@ -62,7 +62,7 @@ protected function handle_authorization_submission( $submit, Client $client, $da
 				);
 		}
 
-		if ( ! empty( $data['state'] ) ) {
+		if ( isset( $data['state'] ) ) {
 			$redirect_args['state'] = $data['state'];
 		}
 
@@ -73,7 +73,7 @@ protected function handle_authorization_submission( $submit, Client $client, $da
 			$data
 		);
 
-		$fragment           = build_query( $redirect_args );
+		$fragment           = build_query( urlencode_deep( $redirect_args ) );
 		$generated_redirect = $redirect_uri . '#' . $fragment;
 		wp_safe_redirect( $generated_redirect );
 		exit;
diff --git a/tests/test-state.php b/tests/test-state.php
new file mode 100644
index 0000000..6ea80d8
--- /dev/null
+++ b/tests/test-state.php
@@ -0,0 +1,124 @@
+<?php
+/**
+ * Tests that the OAuth state parameter is returned to the client unchanged.
+ *
+ * @package WP\OAuth2\Tests
+ */
+
+namespace WP\OAuth2\Tests;
+
+require_once __DIR__ . '/class-test-case.php';
+
+use WP\OAuth2\Client;
+use WP\OAuth2\Types\Authorization_Code;
+use WP\OAuth2\Types\Base;
+use WP\OAuth2\Types\Implicit;
+
+/**
+ * Thrown from a 'wp_redirect' filter to capture the redirect location before exit().
+ */
+class State_Redirect_Interrupt extends \RuntimeException {}
+
+/**
+ * Test cases for state round-tripping in the authorize flow.
+ */
+class Test_State extends Test_Case {
+
+	public function set_up() {
+		parent::set_up();
+		wp_set_current_user( self::factory()->user->create() );
+		add_filter( 'wp_redirect', [ $this, 'interrupt_redirect' ] );
+	}
+
+	public function tear_down() {
+		remove_filter( 'wp_redirect', [ $this, 'interrupt_redirect' ] );
+		$_GET  = [];
+		$_POST = [];
+		parent::tear_down();
+	}
+
+	public function interrupt_redirect( $location ) {
+		throw new State_Redirect_Interrupt( $location );
+	}
+
+	/**
+	 * Approve an authorisation request and return the redirect location.
+	 *
+	 * @param Base   $type Grant type handler.
+	 * @param Client $client Client being authorised.
+	 * @param mixed  $state Raw state value, or null to omit it.
+	 * @return string Redirect location.
+	 */
+	protected function authorize( Base $type, Client $client, $state ) {
+		$_GET = [
+			'client_id'    => $client->get_id(),
+			'redirect_uri' => $client->get_redirect_uris()[0],
+		];
+		if ( null !== $state ) {
+			$_GET['state'] = is_string( $state ) ? wp_slash( $state ) : $state;
+		}
+		$_POST = [
+			'_wpnonce'  => wp_create_nonce( sprintf( 'oauth2_authorize:%s', $client->get_id() ) ),
+			'wp-submit' => 'authorize',
+		];
+
+		try {
+			$type->handle_authorisation();
+		} catch ( State_Redirect_Interrupt $redirect ) {
+			return $redirect->getMessage();
+		}
+
+		$this->fail( 'Expected a redirect.' );
+	}
+
+	public function state_provider() {
+		return [
+			'percent-encoded octets' => [ '%41%42' ],
+			'tag-like text'          => [ '<b>x</b>' ],
+			'zero'                   => [ '0' ],
+			'reserved characters'    => [ 'a&b=c#d' ],
+			'surrounding spaces'     => [ '  padded  ' ],
+			'quotes and slashes'     => [ 'it\'s "a\\b"' ],
+		];
+	}
+
+	/**
+	 * @dataProvider state_provider
+	 */
+	public function test_authorization_code_returns_exact_state( $state ) {
+		$client   = $this->create_client();
+		$location = $this->authorize( new Authorization_Code(), $client, $state );
+
+		parse_str( wp_parse_url( $location, PHP_URL_QUERY ), $args );
+		$this->assertSame( $state, $args['state'] );
+	}
+
+	/**
+	 * @dataProvider state_provider
+	 */
+	public function test_implicit_returns_exact_state( $state ) {
+		// A same-site callback, since the implicit grant redirects with wp_safe_redirect().
+		$client   = $this->create_client( [ 'callback' => home_url( '/callback' ) ] );
+		$location = $this->authorize( new Implicit(), $client, $state );
+
+		parse_str( wp_parse_url( $location, PHP_URL_FRAGMENT ), $args );
+		$this->assertSame( $state, $args['state'] );
+		$this->assertArrayHasKey( 'access_token', $args );
+	}
+
+	public function test_absent_state_is_not_returned() {
+		$client   = $this->create_client();
+		$location = $this->authorize( new Authorization_Code(), $client, null );
+
+		parse_str( wp_parse_url( $location, PHP_URL_QUERY ), $args );
+		$this->assertArrayNotHasKey( 'state', $args );
+	}
+
+	public function test_array_state_is_treated_as_absent() {
+		$client   = $this->create_client();
+		$location = $this->authorize( new Authorization_Code(), $client, [ 'x' ] );
+
+		parse_str( wp_parse_url( $location, PHP_URL_QUERY ), $args );
+		$this->assertArrayNotHasKey( 'state', $args );
+	}
+}