diff --git a/crates/zesdex-iam/src/infrastructure/persistence/mod.rs b/crates/zesdex-iam/src/infrastructure/persistence/mod.rs
index 9e331b2..3389461 100644
--- a/crates/zesdex-iam/src/infrastructure/persistence/mod.rs
+++ b/crates/zesdex-iam/src/infrastructure/persistence/mod.rs
@@ -1,2 +1,3 @@
 pub mod oauth_repo;
+pub mod session_lock_repo;
 pub mod session_repo;
diff --git a/crates/zesdex-iam/src/infrastructure/persistence/session_lock_repo.rs b/crates/zesdex-iam/src/infrastructure/persistence/session_lock_repo.rs
new file mode 100644
index 0000000..49f6509
--- /dev/null
+++ b/crates/zesdex-iam/src/infrastructure/persistence/session_lock_repo.rs
@@ -0,0 +1,147 @@
+//! Filesystem-backed `SessionLockRepository` implementation.
+//!
+//! Ported from `zesdex_entities::seaorm::auth::session_lock::SessionLock`'s
+//! inherent methods — same atomic-create-based locking, same stale-PID
+//! recovery via `libc::kill(pid, 0)` plus a `/proc/<pid>/exe` identity
+//! check to guard against PID reuse. This repository is stateless (no
+//! `Drop`-based auto-release) — callers that need panic-safety should wrap
+//! acquisition in their own RAII guard (see `zesdex-backend`'s
+//! `main.rs::SessionLockGuard`, added in a later task of this plan).
+use std::fs;
+use std::io::Write;
+use std::path::Path;
+
+use crate::domain::repository::SessionLockRepository;
+
+/// Concrete filesystem session-lock repository, using a PID file
+/// (`<session_dir>/.lock`) with atomic `O_CREAT|O_EXCL` acquisition.
+#[derive(Debug, Clone, Default)]
+pub struct FileSystemSessionLockRepository;
+
+impl FileSystemSessionLockRepository {
+    /// Create a new filesystem session-lock repository.
+    pub fn new() -> Self {
+        FileSystemSessionLockRepository
+    }
+}
+
+impl SessionLockRepository for FileSystemSessionLockRepository {
+    fn try_lock(&self, session_dir: &Path) -> anyhow::Result<bool> {
+        let path = session_dir.join(".lock");
+        let pid = std::process::id();
+
+        match fs::OpenOptions::new().create_new(true).write(true).open(&path) {
+            Ok(mut file) => {
+                write!(file, "{pid}")?;
+                file.sync_all()?;
+                return Ok(true);
+            }
+            Err(ref e) if e.kind() == std::io::ErrorKind::AlreadyExists => {}
+            Err(e) => return Err(e.into()),
+        }
+
+        let content = fs::read_to_string(&path).unwrap_or_default();
+        if let Ok(existing_pid) = content.trim().parse::<u32>() {
+            if self.is_alive(existing_pid) {
+                return Ok(false);
+            }
+        }
+
+        let tmp = path.with_extension("lock.tmp");
+        {
+            let mut tmp_file = fs::OpenOptions::new().create(true).truncate(true).write(true).open(&tmp)?;
+            write!(tmp_file, "{pid}")?;
+            tmp_file.sync_all()?;
+        }
+        fs::rename(&tmp, &path)?;
+        if let Some(parent) = path.parent() {
+            let _ = fs::File::open(parent).and_then(|d| d.sync_all());
+        }
+        Ok(true)
+    }
+
+    fn unlock(&self, session_dir: &Path) -> anyhow::Result<()> {
+        let path = session_dir.join(".lock");
+        let _ = fs::remove_file(path);
+        Ok(())
+    }
+
+    fn is_alive(&self, pid: u32) -> bool {
+        // SAFETY: `libc::kill(pid, 0)` sends no signal; it only probes
+        // whether the process exists and is signalable by us.
+        if unsafe { libc::kill(pid as i32, 0) != 0 } {
+            return false;
+        }
+        let proc_exe = std::path::PathBuf::from(format!("/proc/{pid}/exe"));
+        if let Ok(target) = std::fs::read_link(&proc_exe) {
+            if let Ok(exe) = std::env::current_exe() {
+                if target != exe {
+                    return false;
+                }
+            }
+        }
+        true
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn tmp_dir() -> std::path::PathBuf {
+        let dir = std::env::temp_dir().join(format!("zesdex-iam-lock-test-{}", uuid::Uuid::new_v4()));
+        std::fs::create_dir_all(&dir).unwrap();
+        dir
+    }
+
+    #[test]
+    fn try_lock_succeeds_when_no_lock_file_exists() {
+        let dir = tmp_dir();
+        let repo = FileSystemSessionLockRepository::new();
+        assert!(repo.try_lock(&dir).unwrap());
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn try_lock_fails_when_held_by_a_live_process() {
+        let dir = tmp_dir();
+        let repo = FileSystemSessionLockRepository::new();
+        assert!(repo.try_lock(&dir).unwrap());
+        // A second acquisition attempt (simulating our own still-live PID)
+        // must fail since the lock file already holds a live PID.
+        assert!(!repo.try_lock(&dir).unwrap());
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn try_lock_recovers_a_stale_lock() {
+        let dir = tmp_dir();
+        let repo = FileSystemSessionLockRepository::new();
+        // Write a lock file with a PID that cannot possibly be alive.
+        std::fs::write(dir.join(".lock"), "999999999").unwrap();
+        assert!(repo.try_lock(&dir).unwrap(), "a stale lock (dead PID) must be recoverable");
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn unlock_removes_the_lock_file() {
+        let dir = tmp_dir();
+        let repo = FileSystemSessionLockRepository::new();
+        assert!(repo.try_lock(&dir).unwrap());
+        repo.unlock(&dir).unwrap();
+        assert!(!dir.join(".lock").exists());
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn is_alive_returns_true_for_current_process() {
+        let repo = FileSystemSessionLockRepository::new();
+        assert!(repo.is_alive(std::process::id()));
+    }
+
+    #[test]
+    fn is_alive_returns_false_for_implausible_pid() {
+        let repo = FileSystemSessionLockRepository::new();
+        assert!(!repo.is_alive(999_999_999));
+    }
+}