diff --git a/CHANGELOG.md b/CHANGELOG.md
index de0b34c..5e97058 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -145,6 +145,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Added `Installer::verifyChecksum()` public method
   - Added `tests/test_installer_checksum.phpt` — 5 test scenarios covering valid checksum, mismatch, empty file, binary content, and recomputed hash
 
+- **SEC-009: Add advisory file locking (flock) to prevent TOCTOU race condition in concurrent Installer::install()** (#77)
+  - Acquires `LOCK_EX` on `lib/install.lock` before the `file_exists($libPath)` check (lock-check pattern)
+  - Double-check inside the lock: re-verifies the library isn't already installed by another concurrent process
+  - Lock file persists as a sentinel (never deleted) — ensures all concurrent processes share the same inode for `flock()` serialization
+  - Lock is released in `finally` block — crash-safe (kernel auto-releases `flock` on process termination)
+  - Added `tests/test_installer_flock.phpt` — 4 test scenarios covering basic flock, concurrent serialization, source code verification, and behavioral double-check test with `Installer::install()`
+
 ### Changed
 
 - **SMELL-004: Triplicated Insert/Upsert/Update Code** (#85)
diff --git a/src/Installer.php b/src/Installer.php
index cd2ea1c..dd106a0 100644
--- a/src/Installer.php
+++ b/src/Installer.php
@@ -21,8 +21,14 @@ class Installer
      * SHA-256 checksum verification is performed before extraction to ensure integrity
      * of the downloaded archive (see verifyChecksum()).
      *
+     * Concurrent installations are serialized via advisory file locking (flock) to prevent
+     * TOCTOU race conditions between the file-exists check and the download+extract window.
+     * The lock is acquired first, then the existence of the library is re-checked inside the
+     * lock (lock-check pattern). The lock file persists as a sentinel to ensure all concurrent
+     * processes share the same inode for flock() serialization.
+     *
      * @param string|null $version Release version tag (e.g., "v0.4.10"). Auto-detected from composer if null.
-     * @throws RuntimeException On download failure, checksum mismatch, extraction failure, or missing lib in archive.
+     * @throws RuntimeException On download failure, checksum mismatch, extraction failure, lock failure, or missing lib in archive.
      */
     public static function install(?string $version = null): void
     {
@@ -46,38 +52,58 @@ public static function install(?string $version = null): void
 
         $libName = self::libName();
         $libPath = $libDir . '/' . $libName;
-        if (file_exists($libPath)) {
-            echo "zvec FFI library already installed at {$libPath}\n";
-            return;
-        }
 
-        echo "Downloading zvec FFI library {$version} for " . self::platformLabel() . "...\n";
-
-        $tmpDir = sys_get_temp_dir() . '/zvec_ffi_' . bin2hex(random_bytes(8));
-        if (!mkdir($tmpDir, 0700)) {
-            throw new RuntimeException("Failed to create temporary directory");
+        // Acquire exclusive lock to serialize concurrent installations (TOCTOU mitigation)
+        $lockFile = $libDir . '/install.lock';
+        $lockFh = fopen($lockFile, 'w+');
+        if (!$lockFh) {
+            throw new RuntimeException("Could not create lock file: {$lockFile}");
+        }
+        if (!flock($lockFh, LOCK_EX)) {
+            fclose($lockFh);
+            throw new RuntimeException("Could not acquire installation lock");
         }
-        $tmpFile = $tmpDir . '/download.tar.gz';
 
         try {
-            self::download($url, $tmpFile);
+            // Double-check after acquiring lock — another process may have installed it
+            if (file_exists($libPath)) {
+                echo "zvec FFI library already installed at {$libPath}\n";
+                return;
+            }
 
-            $expectedHash = self::getExpectedHash($version, $assetName);
-            self::verifyChecksum($tmpFile, $expectedHash);
+            echo "Downloading zvec FFI library {$version} for " . self::platformLabel() . "...\n";
 
-            self::extract($tmpFile, $libDir);
-        } finally {
-            if (file_exists($tmpFile)) {
-                unlink($tmpFile);
+            $tmpDir = sys_get_temp_dir() . '/zvec_ffi_' . bin2hex(random_bytes(8));
+            if (!mkdir($tmpDir, 0700)) {
+                throw new RuntimeException("Failed to create temporary directory");
             }
-            exec("rm -rf " . escapeshellarg($tmpDir));
-        }
+            $tmpFile = $tmpDir . '/download.tar.gz';
 
-        if (!file_exists($libPath)) {
-            throw new RuntimeException("Download succeeded but {$libName} not found in archive.");
-        }
+            try {
+                self::download($url, $tmpFile);
 
-        echo "zvec FFI library installed at {$libPath}\n";
+                $expectedHash = self::getExpectedHash($version, $assetName);
+                self::verifyChecksum($tmpFile, $expectedHash);
+
+                self::extract($tmpFile, $libDir);
+            } finally {
+                if (file_exists($tmpFile)) {
+                    unlink($tmpFile);
+                }
+                exec("rm -rf " . escapeshellarg($tmpDir));
+            }
+
+            if (!file_exists($libPath)) {
+                throw new RuntimeException("Download succeeded but {$libName} not found in archive.");
+            }
+
+            echo "zvec FFI library installed at {$libPath}\n";
+        } finally {
+            flock($lockFh, LOCK_UN);
+            fclose($lockFh);
+            // Lock file persists as a sentinel. Never delete — removing it would let
+            // a new process create a different inode and bypass flock() serialization.
+        }
     }
 
     public static function verifyChecksum(string $filePath, string $expectedHash): void
diff --git a/tests/test_installer_flock.phpt b/tests/test_installer_flock.phpt
new file mode 100644
index 0000000..0739b6c
--- /dev/null
+++ b/tests/test_installer_flock.phpt
@@ -0,0 +1,175 @@
+--TEST--
+SEC-009: flock() prevents TOCTOU race condition in Installer::install()
+--SKIPIF--
+<?php if (PHP_OS_FAMILY === 'Windows') die('skip flock test not applicable on Windows'); ?>
+--FILE--
+<?php
+declare(strict_types=1);
+require_once __DIR__ . '/../src/ZVec.php';
+require_once __DIR__ . '/../src/Installer.php';
+
+use CrazyGoat\ZVec\Installer;
+
+$path = __DIR__ . '/../test_dbs/installer_flock_' . uniqid();
+try {
+    // --- Test 1: Verify lock file is created and cleaned up ---
+    $lockDir = sys_get_temp_dir() . '/zvec_flock_test_' . bin2hex(random_bytes(8));
+    mkdir($lockDir, 0755, true);
+
+    $lockFile = $lockDir . '/install.lock';
+    $lockFh = fopen($lockFile, 'w+');
+    if (!$lockFh) {
+        echo "FAIL: Could not create lock file\n";
+        exit(1);
+    }
+
+    $locked = flock($lockFh, LOCK_EX);
+    echo "Lock acquired: " . ($locked ? 'yes' : 'no') . "\n";
+    echo "Lock file exists: " . (file_exists($lockFile) ? 'yes' : 'no') . "\n";
+
+    flock($lockFh, LOCK_UN);
+    fclose($lockFh);
+    @unlink($lockFile);
+    echo "Lock file after cleanup: " . (file_exists($lockFile) ? 'yes' : 'no') . "\n";
+
+    exec("rm -rf " . escapeshellarg($lockDir));
+
+    // --- Test 2: Concurrent lock acquisition is serialized ---
+    $lockDir = sys_get_temp_dir() . '/zvec_flock_concurrent_' . bin2hex(random_bytes(8));
+    mkdir($lockDir, 0755, true);
+    $lockFile = $lockDir . '/install.lock';
+    $logFile = $lockDir . '/log.txt';
+    file_put_contents($logFile, '');
+
+    $procs = [];
+    for ($i = 0; $i < 3; $i++) {
+        $code = <<<'PHP'
+<?php
+$lockFile = $argv[1];
+$logFile = $argv[2];
+$id = $argv[3];
+
+$fh = fopen($lockFile, 'w+');
+if (!$fh) exit(1);
+
+if (flock($fh, LOCK_EX)) {
+    file_put_contents($logFile, "START $id\n", FILE_APPEND);
+    usleep(50000);
+    file_put_contents($logFile, "END $id\n", FILE_APPEND);
+    flock($fh, LOCK_UN);
+}
+fclose($fh);
+PHP;
+        $tmpScript = $lockDir . "/child_{$i}.php";
+        file_put_contents($tmpScript, $code);
+        $procs[] = proc_open(
+            "php " . escapeshellarg($tmpScript) . " " . escapeshellarg($lockFile) . " " . escapeshellarg($logFile) . " $i",
+            [STDIN, STDOUT, STDERR],
+            $pipes
+        );
+    }
+
+    foreach ($procs as $proc) {
+        proc_close($proc);
+    }
+
+    $lines = array_filter(explode("\n", trim(file_get_contents($logFile))));
+    echo "Log entries: " . count($lines) . "\n";
+
+    $active = 0;
+    $maxActive = 0;
+    $overlapped = false;
+    foreach ($lines as $line) {
+        if (str_starts_with($line, 'START')) {
+            $active++;
+            if ($active > 1) {
+                $overlapped = true;
+            }
+            $maxActive = max($maxActive, $active);
+        } elseif (str_starts_with($line, 'END')) {
+            $active--;
+        }
+    }
+
+    echo "Max concurrent locks: $maxActive\n";
+    echo "Overlapped: " . ($overlapped ? 'yes' : 'no') . "\n";
+
+    // --- Test 3: Verify flock() code exists in Installer.php ---
+    $source = file_get_contents(__DIR__ . '/../src/Installer.php');
+    $hasFlock = str_contains($source, 'flock($lockFh, LOCK_EX)');
+    $hasUnlock = str_contains($source, 'flock($lockFh, LOCK_UN)');
+    $hasLockFile = str_contains($source, "install.lock");
+    $hasDoubleCheck = substr_count($source, 'file_exists($libPath)') >= 2;
+
+    echo "flock LOCK_EX in source: " . ($hasFlock ? 'yes' : 'no') . "\n";
+    echo "flock LOCK_UN in source: " . ($hasUnlock ? 'yes' : 'no') . "\n";
+    echo "install.lock in source: " . ($hasLockFile ? 'yes' : 'no') . "\n";
+    echo "double-check pattern: " . ($hasDoubleCheck ? 'yes' : 'no') . "\n";
+
+    exec("rm -rf " . escapeshellarg($lockDir));
+
+    // --- Test 4: Behavioral — double-check pattern skips download when lib already exists ---
+    echo "---\n";
+    $realLibPath = __DIR__ . '/../lib/' . (PHP_OS_FAMILY === 'Darwin' ? 'libzvec_ffi.dylib' : 'libzvec_ffi.so');
+    $dummyCreated = false;
+    try {
+        // Create dummy library file so Installer::install() sees it as already installed
+        file_put_contents($realLibPath, 'dummy');
+        $dummyCreated = true;
+
+        // This should detect the file and return early (no download, no exception)
+        ob_start();
+        Installer::install('v0.4.0');
+        $output = ob_get_clean();
+
+        if (str_contains($output, 'already installed')) {
+            echo "Test 4: Double-check detected existing library\n";
+        } else {
+            echo "FAIL: Expected 'already installed' message\n";
+            exit(1);
+        }
+
+        // Verify lock file persists as sentinel (never deleted — ensures same inode for flock)
+        $libDir = __DIR__ . '/../lib';
+        $lockFilePath = $libDir . '/install.lock';
+        if (file_exists($lockFilePath)) {
+            echo "Test 4: Lock file persists as sentinel\n";
+        } else {
+            echo "FAIL: Lock file was deleted (sentinel should persist)\n";
+            exit(1);
+        }
+
+        echo "Test 4 PASS\n";
+    } finally {
+        // Clean up dummy library
+        if ($dummyCreated && file_exists($realLibPath)) {
+            unlink($realLibPath);
+        }
+        // Clean up lock file left by test (Installer's own finally never deletes it)
+        $lockFilePath = __DIR__ . '/../lib/install.lock';
+        if (file_exists($lockFilePath)) {
+            unlink($lockFilePath);
+        }
+    }
+
+    echo "DONE\n";
+} finally {
+    exec("rm -rf " . escapeshellarg($path));
+}
+?>
+--EXPECT--
+Lock acquired: yes
+Lock file exists: yes
+Lock file after cleanup: no
+Log entries: 6
+Max concurrent locks: 1
+Overlapped: no
+flock LOCK_EX in source: yes
+flock LOCK_UN in source: yes
+install.lock in source: yes
+double-check pattern: yes
+---
+Test 4: Double-check detected existing library
+Test 4: Lock file persists as sentinel
+Test 4 PASS
+DONE