diff --git a/tests/phpunit/tests/admin/includes/plugin/activatePlugin.php b/tests/phpunit/tests/admin/includes/plugin/activatePlugin.php
new file mode 100644
index 0000000000000..4f4c7c93b2979
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/plugin/activatePlugin.php
@@ -0,0 +1,325 @@
+<?php
+
+/**
+ * Tests for activate_plugin().
+ *
+ * @group admin
+ * @group plugins
+ *
+ * @covers ::activate_plugin
+ */
+class Tests_Admin_Includes_Plugin_ActivatePlugin extends WP_UnitTestCase {
+
+	/**
+	 * Number used to give each test plugin a unique slug, since plugin files are included once.
+	 *
+	 * @var int
+	 */
+	private static $counter = 0;
+
+	/**
+	 * The slug of the test plugin.
+	 *
+	 * @var string
+	 */
+	private $slug;
+
+	/**
+	 * Whether $_GET['networkwide'] was set before the test.
+	 *
+	 * @var bool
+	 */
+	private $had_networkwide;
+
+	/**
+	 * The redirect locations passed to wp_redirect().
+	 *
+	 * @var string[]
+	 */
+	private $redirects = array();
+
+	/**
+	 * Resets the active plugins and prepares a unique test plugin.
+	 */
+	public function set_up() {
+		parent::set_up();
+
+		update_option( 'active_plugins', array( 'zzz/zzz.php' ) );
+		update_site_option( 'active_sitewide_plugins', array() );
+
+		$this->slug            = 'activation-test-plugin-' . ++self::$counter;
+		$this->had_networkwide = array_key_exists( 'networkwide', $_GET );
+		$this->redirects       = array();
+
+		add_filter( 'wp_redirect', array( $this, 'record_redirect' ) );
+	}
+
+	/**
+	 * Removes the test plugin and restores $_GET.
+	 */
+	public function tear_down() {
+		$this->remove_plugin();
+		wp_cache_delete( 'plugins', 'plugins' );
+
+		if ( ! $this->had_networkwide ) {
+			unset( $_GET['networkwide'] );
+		}
+
+		parent::tear_down();
+	}
+
+	/**
+	 * Tests that a plugin that is not installed is not activated.
+	 *
+	 * On multisite, is_network_only_plugin() reads the missing file before the plugin is validated.
+	 *
+	 * @ticket 65819
+	 *
+	 * @group ms-excluded
+	 */
+	public function test_should_reject_plugin_that_is_not_installed() {
+		$result = activate_plugin( 'missing/missing.php' );
+
+		$this->assertWPError( $result, 'An error should be returned.' );
+		$this->assertSame( 'plugin_not_found', $result->get_error_code(), 'The error code should match.' );
+		$this->assertSame( array( 'zzz/zzz.php' ), get_option( 'active_plugins' ), 'The active plugins should not change.' );
+	}
+
+	/**
+	 * Tests that a plugin whose requirements are not met is not activated.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_reject_plugin_with_unmet_requirements() {
+		$this->create_plugin( " * Requires PHP: 99.0\n" );
+
+		$result = activate_plugin( $this->get_file() );
+
+		$this->assertWPError( $result, 'An error should be returned.' );
+		$this->assertSame( 'plugin_php_incompatible', $result->get_error_code(), 'The error code should match.' );
+		$this->assertSame( array( 'zzz/zzz.php' ), get_option( 'active_plugins' ), 'The active plugins should not change.' );
+	}
+
+	/**
+	 * Tests that a plugin is activated, sorted into the active plugins, and the activation hooks fire.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_activate_plugin_and_fire_hooks() {
+		$this->create_plugin();
+
+		$actions = $this->watch_activation_hooks();
+
+		$this->assertNull( activate_plugin( $this->get_file() ), 'Nothing should be returned.' );
+		$this->assertSame( array( $this->get_file(), 'zzz/zzz.php' ), get_option( 'active_plugins' ), 'The plugin should be added in order.' );
+		$this->assertSame(
+			array(
+				array( 'activate_plugin', array( $this->get_file(), false ) ),
+				array( "activate_{$this->get_file()}", array( false ) ),
+				array( 'activated_plugin', array( $this->get_file(), false ) ),
+			),
+			$actions->calls,
+			'The activation hooks should fire in order.'
+		);
+	}
+
+	/**
+	 * Tests that the plugin path is reduced to its basename.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_accept_full_path_with_whitespace() {
+		$this->create_plugin();
+
+		$this->assertNull( activate_plugin( '  ' . WP_PLUGIN_DIR . '/' . $this->get_file() . ' ' ), 'Nothing should be returned.' );
+		$this->assertContains( $this->get_file(), get_option( 'active_plugins' ), 'The plugin should be active.' );
+	}
+
+	/**
+	 * Tests that no hooks fire in silent mode.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_not_fire_hooks_in_silent_mode() {
+		$this->create_plugin();
+
+		$actions = $this->watch_activation_hooks();
+
+		$this->assertNull( activate_plugin( $this->get_file(), '', false, true ), 'Nothing should be returned.' );
+		$this->assertContains( $this->get_file(), get_option( 'active_plugins' ), 'The plugin should be active.' );
+		$this->assertSame( array(), $actions->calls, 'No activation hooks should fire.' );
+	}
+
+	/**
+	 * Tests that an active plugin is not activated again.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_not_activate_active_plugin_again() {
+		$this->create_plugin();
+		update_option( 'active_plugins', array( $this->get_file() ) );
+
+		$actions = $this->watch_activation_hooks();
+
+		$this->assertNull( activate_plugin( $this->get_file(), 'plugins.php' ), 'Nothing should be returned.' );
+		$this->assertSame( array( $this->get_file() ), get_option( 'active_plugins' ), 'The active plugins should not change.' );
+		$this->assertSame( array(), $actions->calls, 'No activation hooks should fire.' );
+		$this->assertSame( array(), $this->redirects, 'No redirect should be sent.' );
+	}
+
+	/**
+	 * Tests that output while loading the plugin is reported, after the plugin is activated.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_report_unexpected_output() {
+		$this->create_plugin( '', "echo 'Unexpected!';\n" );
+
+		$result = activate_plugin( $this->get_file() );
+
+		$this->assertWPError( $result, 'An error should be returned.' );
+		$this->assertSame( 'unexpected_output', $result->get_error_code(), 'The error code should match.' );
+		$this->assertSame( 'The plugin generated unexpected output.', $result->get_error_message(), 'The error message should match.' );
+		$this->assertSame( 'Unexpected!', $result->get_error_data(), 'The output should be the error data.' );
+		$this->assertContains( $this->get_file(), get_option( 'active_plugins' ), 'The plugin should still be activated.' );
+	}
+
+	/**
+	 * Tests that a redirect to the error page is sent before the plugin is loaded.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_redirect_with_error_nonce() {
+		$this->create_plugin();
+
+		activate_plugin( $this->get_file(), 'plugins.php?error=true&plugin=' . $this->get_file() );
+
+		$this->assertSame(
+			array( "plugins.php?error=true&plugin={$this->slug}%2F{$this->slug}.php&_error_nonce=" . wp_create_nonce( 'plugin-activation-error_' . $this->get_file() ) ),
+			$this->redirects
+		);
+	}
+
+	/**
+	 * Tests that a plugin is activated network-wide on multisite.
+	 *
+	 * @ticket 65819
+	 *
+	 * @group ms-required
+	 */
+	public function test_should_activate_plugin_network_wide() {
+		$this->create_plugin();
+
+		$actions = $this->watch_activation_hooks();
+
+		$this->assertNull( activate_plugin( $this->get_file(), '', true ), 'Nothing should be returned.' );
+		$this->assertSame( array( $this->get_file() ), array_keys( get_site_option( 'active_sitewide_plugins' ) ), 'The plugin should be network active.' );
+		$this->assertSame( array( 'zzz/zzz.php' ), get_option( 'active_plugins' ), 'The site plugins should not change.' );
+		$this->assertSame( 1, $_GET['networkwide'], 'The back-compat request value should be set.' );
+		$this->assertSame( array( 'activate_plugin', array( $this->get_file(), true ) ), $actions->calls[0], 'The hooks should receive true.' );
+	}
+
+	/**
+	 * Tests that a network-only plugin is always activated network-wide on multisite.
+	 *
+	 * @ticket 65819
+	 *
+	 * @group ms-required
+	 */
+	public function test_should_activate_network_only_plugin_network_wide() {
+		$this->create_plugin( " * Network: true\n" );
+
+		$this->assertNull( activate_plugin( $this->get_file() ), 'Nothing should be returned.' );
+		$this->assertArrayHasKey( $this->get_file(), get_site_option( 'active_sitewide_plugins' ), 'The plugin should be network active.' );
+	}
+
+	/**
+	 * Tests that a network-active plugin is not activated again.
+	 *
+	 * @ticket 65819
+	 *
+	 * @group ms-required
+	 */
+	public function test_should_not_activate_network_active_plugin_again() {
+		$this->create_plugin();
+		update_site_option( 'active_sitewide_plugins', array( $this->get_file() => 1000 ) );
+
+		$this->assertNull( activate_plugin( $this->get_file(), '', true ), 'Nothing should be returned.' );
+		$this->assertSame( array( $this->get_file() => 1000 ), get_site_option( 'active_sitewide_plugins' ), 'The activation time should not change.' );
+	}
+
+	/**
+	 * Records redirect locations and stops the redirect.
+	 *
+	 * @param string $location The redirect location.
+	 * @return false False, so that no header is sent.
+	 */
+	public function record_redirect( $location ) {
+		$this->redirects[] = $location;
+
+		return false;
+	}
+
+	/**
+	 * Records the activation hooks.
+	 *
+	 * @return object An object whose $calls property lists the hook names and arguments.
+	 */
+	private function watch_activation_hooks() {
+		$recorder        = new stdClass();
+		$recorder->calls = array();
+
+		foreach ( array( 'activate_plugin', "activate_{$this->get_file()}", 'activated_plugin' ) as $hook ) {
+			add_action(
+				$hook,
+				static function ( ...$args ) use ( $recorder, $hook ) {
+					$recorder->calls[] = array( $hook, $args );
+				},
+				10,
+				2
+			);
+		}
+
+		return $recorder;
+	}
+
+	/**
+	 * Returns the plugin file of the test plugin.
+	 *
+	 * @return string The plugin file.
+	 */
+	private function get_file() {
+		return "{$this->slug}/{$this->slug}.php";
+	}
+
+	/**
+	 * Creates the test plugin.
+	 *
+	 * @param string $headers Additional plugin headers.
+	 * @param string $code    Code after the headers.
+	 */
+	private function create_plugin( $headers = '', $code = '' ) {
+		$dir = WP_PLUGIN_DIR . '/' . $this->slug;
+
+		$this->remove_plugin();
+		$this->assertTrue( mkdir( $dir ), 'The test plugin directory should be created.' );
+		$this->assertNotFalse(
+			file_put_contents( "{$dir}/{$this->slug}.php", "<?php\n/**\n * Plugin Name: Activation Test Plugin\n{$headers} */\n{$code}" ),
+			'The test plugin should be written.'
+		);
+
+		wp_cache_delete( 'plugins', 'plugins' );
+	}
+
+	/**
+	 * Removes the test plugin, including one left over by an interrupted run.
+	 */
+	private function remove_plugin() {
+		$dir = WP_PLUGIN_DIR . '/' . $this->slug;
+
+		if ( is_dir( $dir ) ) {
+			$this->rmdir( $dir );
+			rmdir( $dir );
+		}
+	}
+}