diff -u b/core/modules/update/tests/src/Unit/UpdateFetcherTest.php b/core/modules/update/tests/src/Unit/UpdateFetcherTest.php
--- b/core/modules/update/tests/src/Unit/UpdateFetcherTest.php
+++ b/core/modules/update/tests/src/Unit/UpdateFetcherTest.php
@@ -5,10 +5,17 @@
 use Drupal\Core\Site\Settings;
 use Drupal\Tests\UnitTestCase;
 use Drupal\update\UpdateFetcher;
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Middleware;
+use GuzzleHttp\Psr7\Response;
 
 /**
  * Tests update functionality unrelated to the database.
  *
+ * @coversDefaultClass \Drupal\update\UpdateFetcher
+ *
  * @group update
  */
 class UpdateFetcherTest extends UnitTestCase {
@@ -21,13 +28,51 @@
   protected $updateFetcher;
 
   /**
+   * History of requests/responses.
+   *
+   * @var array
+   */
+  protected $history = [];
+
+  /**
+   * Mock HTTP client.
+   *
+   * @var \GuzzleHttp\ClientInterface
+   */
+  protected $mockHttpClient;
+
+  /**
+   * Mock config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $mockConfigFactory;
+
+  /**
+   * A test project to fetch with.
+   *
+   * @var array
+   */
+  protected $testProject;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUp(): void {
-    $config_factory = $this->getConfigFactoryStub(['update.settings' => ['fetch_url' => 'http://www.example.com']]);
-    $http_client_mock = $this->createMock('\GuzzleHttp\ClientInterface');
+    parent::setUp();
+    $this->mockConfigFactory = $this->getConfigFactoryStub(['update.settings' => ['fetch_url' => 'http://www.example.com']]);
+    $this->mockHttpClient = $this->createMock('\GuzzleHttp\ClientInterface');
     $settings = new Settings([]);
-    $this->updateFetcher = new UpdateFetcher($config_factory, $http_client_mock, $settings);
+    $this->updateFetcher = new UpdateFetcher($this->mockConfigFactory, $this->mockHttpClient, $settings);
+    $this->testProject = [
+      'name' => 'update_test',
+      'project_type' => '',
+      'info' => [
+        'version' => '',
+        'project status url' => 'https://www.example.com',
+      ],
+      'includes' => ['module1' => 'Module 1', 'module2' => 'Module 2'],
+    ];
   }
 
   /**
@@ -101,2 +146,92 @@
 
+  /**
+   * Mocks the HTTP client.
+   *
+   * @param GuzzleHttp\Psr7\Response ...$responses
+   *   Variable number of Response objects that the mocked client should return.
+   */
+  protected function mockClient(Response ...$responses) {
+    // Create a mock and queue responses.
+    $mock_handler = new MockHandler($responses);
+    $handler_stack = HandlerStack::create($mock_handler);
+    $history = Middleware::history($this->history);
+    $handler_stack->push($history);
+    $this->mockHttpClient = new Client(['handler' => $handler_stack]);
+  }
+
+  /**
+   * @covers ::doRequest
+   * @covers ::fetchProjectData
+   */
+  public function testUpdateFetcherNoFallback() {
+    // First, try without the HTTP fallback setting, and HTTPS mocked to fail.
+    $settings = new Settings([]);
+    $this->mockClient(
+      new Response('500', [], 'https failed'),
+    );
+    $update_fetcher = new UpdateFetcher($this->mockConfigFactory, $this->mockHttpClient, $settings);
+
+    $data = $update_fetcher->fetchProjectData($this->testProject, '');
+    // There should only be one request / response pair.
+    $this->assertCount(1, $this->history);
+    $request = $this->history[0]['request'];
+    $this->assertNotEmpty($request);
+    // It should have only been an https request.
+    $this->assertEquals('https', $request->getUri()->getScheme());
+    // And it should have failed.
+    $response = $this->history[0]['response'];
+    $this->assertEquals(500, $response->getStatusCode());
+    $this->assertEmpty($data);
+  }
+
+  /**
+   * @covers ::doRequest
+   * @covers ::fetchProjectData
+   */
+  public function testUpdateFetcherHttpFallback() {
+    $settings = new Settings(['update_fetch_with_http_fallback' => TRUE]);
+    $this->mockClient(
+      new Response('500', [], 'https failed'),
+      new Response('200', [], 'http worked'),
+    );
+    $update_fetcher = new UpdateFetcher($this->mockConfigFactory, $this->mockHttpClient, $settings);
+
+    $data = $update_fetcher->fetchProjectData($this->testProject, '');
+
+    // There should be two request / response pairs.
+    $this->assertCount(2, $this->history);
+
+    // The first should have been https and should have failed.
+    $first_try = $this->history[0];
+    $this->assertNotEmpty($first_try);
+    $this->assertEquals('https', $first_try['request']->getUri()->getScheme());
+    $this->assertEquals(500, $first_try['response']->getStatusCode());
+
+    // The second should have been the http fallback and should have worked.
+    $second_try = $this->history[1];
+    $this->assertNotEmpty($second_try);
+    $this->assertEquals('http', $second_try['request']->getUri()->getScheme());
+    $this->assertEquals(200, $second_try['response']->getStatusCode());
+    // Although this is a bogus mocked response, it's what fetchProjectData()
+    // should return in this case.
+    $this->assertEquals('http worked', $data);
+  }
+
+}
+
+namespace Drupal\update;
+use Drupal\Core\Logger\RfcLogLevel;
+/**
+ * Mock version of watchdog_exception().
+ *
+ * This is unfortunate, but UpdateFetcher::doRequest() calls
+ * watchdog_exception(), which is defined in includes/bootstrap.inc, which
+ * cannot be included for Unit tests or we break unit test encapsulation.
+ * Therefore, we define our own copy in the \Drupal\update namespace that
+ * doesn't do anything (we're not trying to test the exception logging).
+ *
+ * @todo Remove in https://www.drupal.org/project/drupal/issues/2932518
+ */
+function watchdog_exception($type, \Exception $exception, $message = NULL, $variables = [], $severity = RfcLogLevel::ERROR, $link = NULL) {
+  // No-op.
 }