diff --git a/CHANGELOG.md b/CHANGELOG.md
index e4427a8..1d9bfe6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -62,9 +62,9 @@ deprecated until 3.0.
     by default, configured through its constructor (git binary, environment, timeout and logger,
     which gets every command, clones included), or any `Driver\DriverInterface`, to replace it in
     tests; see ["Using your own driver"](doc/repository.md#using-your-own-driver). The driver
-    finds the repository: linked worktrees, submodules and reftable repositories open, and a
-    directory that isn't a repository throws right away.
-    ([#253](https://github.com/gitonomy/gitlib/pull/253))
+    finds the repository: linked worktrees, submodules, clones with `--separate-git-dir` and
+    reftable repositories open, and a directory that isn't a repository throws right away.
+    ([#253](https://github.com/gitonomy/gitlib/pull/253), fixes #200)
 -   Add static methods to `Repository` creating repositories, `init()`, `initBare()`,
     `cloneFrom()`, `cloneBareFrom()` and `mirrorFrom()`, and checking a repository URL,
     `isValidRepository()` and `isValidRepositoryAndBranch()`; see
diff --git a/UPGRADE-2.0.md b/UPGRADE-2.0.md
index e44b082..d308148 100644
--- a/UPGRADE-2.0.md
+++ b/UPGRADE-2.0.md
@@ -67,7 +67,8 @@ Opening a repository
     ignored.
 *   The `debug` option is deprecated, and setting it to `false` throws an
     `InvalidArgumentException`: see [Errors](#errors).
-*   `$path` may be a linked worktree or a submodule, whose `.git` is a file.
+*   `$path` may be a linked worktree, a submodule or a clone with
+    `--separate-git-dir`, whose `.git` is a file.
 *   A directory that isn't a repository throws an `InvalidArgumentException`
     right away, instead of a `ProcessException` at the first operation.
 
diff --git a/doc/repository.md b/doc/repository.md
index 998a6ad..1eb3277 100644
--- a/doc/repository.md
+++ b/doc/repository.md
@@ -23,7 +23,8 @@ The path may be:
 -   a bare repository;
 -   a working tree, with its `.git` directory;
 -   a working tree whose `.git` is a file pointing to its git directory: a
-    linked worktree (`git worktree add`) or a submodule.
+    linked worktree (`git worktree add`), a submodule, or a clone with
+    `--separate-git-dir`.
 
 The constructor takes two more arguments:
 
diff --git a/src/Gitonomy/Git/Driver/RepositoryPaths.php b/src/Gitonomy/Git/Driver/RepositoryPaths.php
index 7cede83..71141e7 100644
--- a/src/Gitonomy/Git/Driver/RepositoryPaths.php
+++ b/src/Gitonomy/Git/Driver/RepositoryPaths.php
@@ -35,7 +35,7 @@ private function __construct(
      * @param string|null $workingDir The working tree of the git directory $dir
      *
      * @throws InvalidArgumentException      $dir doesn't exist, or isn't a repository, or a path contains a NUL byte
-     * @throws DriverException               The ".git" file of $dir doesn't point to a git directory
+     * @throws DriverException               The ".git" file of $dir can't be read, or doesn't point to a git directory
      * @throws UnsupportedOperationException The repository uses SHA-256 hashes
      */
     public static function resolve(string $dir, ?string $workingDir): self
@@ -50,7 +50,7 @@ public static function resolve(string $dir, ?string $workingDir): self
         $paths = match (true) {
             null !== $workingDir => new self($realDir, $workingDir),
             is_dir($realDir.'/.git') => new self($realDir.'/.git', $realDir),
-            // In a linked worktree or a submodule, ".git" is a file pointing to the git directory.
+            // In a linked worktree, a submodule or a clone with --separate-git-dir, ".git" is a file pointing to the git directory.
             is_file($realDir.'/.git') => new self(self::readGitFile($realDir), $realDir),
             default => new self($realDir, null),
         };
@@ -69,17 +69,29 @@ public static function resolve(string $dir, ?string $workingDir): self
     }
 
     /**
+     * Reads the file like git: "gitdir: " then the path, absolute or relative
+     * to $dir, without the line ending, "\r\n" included. The path may end with
+     * a space.
+     *
      * @return string The git directory the ".git" file of $dir points to
+     *
+     * @throws DriverException The file can't be read, or doesn't point to a git directory
      */
     private static function readGitFile(string $dir): string
     {
-        $content = file_get_contents($dir.'/.git');
-        if (false === $content || !preg_match('/^gitdir: (.+)$/m', $content, $match)) {
+        $content = @file_get_contents($dir.'/.git');
+        if (false === $content) {
+            throw new DriverException(\sprintf('Unable to read the "%s/.git" file.', $dir));
+        }
+
+        $path = rtrim(substr($content, \strlen('gitdir: ')), "\r\n");
+        // realpath() refuses a NUL byte, where git would read the path up to it.
+        if (!str_starts_with($content, 'gitdir: ') || '' === $path || str_contains($path, "\0")) {
             throw new DriverException(\sprintf('The "%s/.git" file doesn\'t point to a git directory.', $dir));
         }
 
-        $gitDir = str_starts_with($match[1], '/') || preg_match('/^[a-zA-Z]:[\\\\\/]/', $match[1]) ? $match[1] : $dir.'/'.$match[1];
+        $gitDir = str_starts_with($path, '/') || preg_match('/^[a-zA-Z]:[\\\\\/]/', $path) ? $path : $dir.'/'.$path;
 
-        return realpath($gitDir) ?: throw new DriverException(\sprintf('The git directory "%s" of "%s" does not exist.', $match[1], $dir));
+        return realpath($gitDir) ?: throw new DriverException(\sprintf('The git directory "%s" of "%s" does not exist.', $path, $dir));
     }
 }
diff --git a/tests/Gitonomy/Git/Tests/RepositoryTest.php b/tests/Gitonomy/Git/Tests/RepositoryTest.php
index 50fd678..2900f1c 100644
--- a/tests/Gitonomy/Git/Tests/RepositoryTest.php
+++ b/tests/Gitonomy/Git/Tests/RepositoryTest.php
@@ -79,6 +79,115 @@ public function testOpensALinkedWorktree(): void
         $this->assertSame($repository->getHeadCommit()->getHash(), $worktree->getHeadCommit()->getHash());
     }
 
+    /**
+     * The ".git" file of a submodule points to its git directory, in the one
+     * of the superproject, with a relative path.
+     */
+    public function testOpensASubmodule(): void
+    {
+        $superproject = self::createFoobarRepository(false);
+        $source = self::createFoobarRepository();
+        // git refuses to clone a local submodule since 2.38.1, unless protocol.file.allow allows it.
+        new Process([self::gitCommand(), '-c', 'protocol.file.allow=always', 'submodule', 'add', '-q', $source->getGitDir(), 'sub'], $superproject->getWorkingDir(), self::gitEnvs())->mustRun();
+        $dir = $superproject->getWorkingDir().'/sub';
+        $this->assertFileMatchesFormat('gitdir: ../.git/modules/sub', $dir.'/.git');
+
+        $submodule = new Repository($dir, driver: self::getDriver());
+
+        $this->assertSame(realpath($dir), $submodule->getWorkingDir());
+        $this->assertSame(realpath($superproject->getGitDir().'/modules/sub'), $submodule->getGitDir());
+        $this->assertFalse($submodule->isBare());
+        $this->assertSame($source->getHeadCommit()->getHash(), $submodule->getHeadCommit()->getHash());
+    }
+
+    /**
+     * "git clone --separate-git-dir" writes a ".git" file with the absolute
+     * path of the git directory.
+     */
+    public function testOpensACloneWithASeparateGitDir(): void
+    {
+        $source = self::createFoobarRepository();
+        [$dir, $gitDir] = self::cloneWithASeparateGitDir($source);
+
+        $repository = new Repository($dir, driver: self::getDriver());
+
+        $this->assertSame(realpath($dir), $repository->getWorkingDir());
+        $this->assertSame(realpath($gitDir), $repository->getGitDir());
+        $this->assertFalse($repository->isBare());
+        $this->assertSame($source->getHeadCommit()->getHash(), $repository->getHeadCommit()->getHash());
+    }
+
+    /**
+     * Like git, which leaves out the line ending of a ".git" file only: a
+     * path may end with a space.
+     */
+    #[DataProvider('provideLineEndings')]
+    public function testOpensAGitFileWhateverItsLineEnding(string $lineEnding): void
+    {
+        [$dir, $gitDir] = self::cloneWithASeparateGitDir(self::createFoobarRepository());
+        file_put_contents($dir.'/.git', 'gitdir: '.$gitDir.$lineEnding);
+
+        $this->assertSame(realpath($gitDir), new Repository($dir, driver: self::getDriver())->getGitDir());
+    }
+
+    /**
+     * @return iterable<string, array{string}>
+     */
+    public static function provideLineEndings(): iterable
+    {
+        yield 'none' => [''];
+        yield 'LF' => ["\n"];
+        yield 'CRLF, written on Windows' => ["\r\n"];
+    }
+
+    #[DataProvider('provideGitFilesPointingToNoGitDirectory')]
+    public function testAGitFilePointingToNoGitDirectoryThrows(string $content, string $message): void
+    {
+        $dir = self::createTempDir();
+        file_put_contents($dir.'/.git', $content);
+
+        $this->expectException(DriverException::class);
+        $this->expectExceptionMessage(\sprintf($message, $dir));
+
+        new Repository($dir, driver: self::getDriver());
+    }
+
+    /**
+     * @return iterable<string, array{string, string}>
+     */
+    public static function provideGitFilesPointingToNoGitDirectory(): iterable
+    {
+        $invalid = 'The "%s/.git" file doesn\'t point to a git directory.';
+
+        yield 'another format' => ["ref: refs/heads/master\n", $invalid];
+        // Like git, which reads "gitdir: " at the start of the file only.
+        yield 'gitdir on the second line' => ["\ngitdir: missing\n", $invalid];
+        yield 'no path' => ["gitdir: \n", $invalid];
+        yield 'NUL byte' => ["gitdir: a\0b\n", $invalid];
+        yield 'missing directory' => ["gitdir: missing\n", 'The git directory "missing" of "%s" does not exist.'];
+    }
+
+    public function testAnUnreadableGitFileThrows(): void
+    {
+        $dir = self::createTempDir();
+        file_put_contents($dir.'/.git', "gitdir: missing\n");
+        chmod($dir.'/.git', 0);
+
+        try {
+            if (is_readable($dir.'/.git')) {
+                $this->markTestSkipped('The user of the tests reads any file, like root.');
+            }
+
+            $this->expectException(DriverException::class);
+            $this->expectExceptionMessage(\sprintf('Unable to read the "%s/.git" file.', $dir));
+
+            new Repository($dir, driver: self::getDriver());
+        } finally {
+            // Or the file can't be deleted on Windows.
+            chmod($dir.'/.git', 0o600);
+        }
+    }
+
     public function testOpeningADirectoryThatIsNotARepositoryThrows(): void
     {
         $dir = self::createTempDir();
@@ -820,4 +929,16 @@ public function testDebugOptionSetToFalseIsRefused(): void
 
         new Repository(self::createTempDir(), ['debug' => false]);
     }
+
+    /**
+     * @return array{string, string} The working tree of the clone, and its git directory
+     */
+    private static function cloneWithASeparateGitDir(Repository $source): array
+    {
+        $dir = self::createTempDir().'/clone';
+        $gitDir = self::createTempDir().'/git';
+        new Process([self::gitCommand(), 'clone', '-q', '--separate-git-dir='.$gitDir, $source->getGitDir(), $dir], null, self::gitEnvs())->mustRun();
+
+        return [$dir, $gitDir];
+    }
 }