diff --git a/cubbi/session.py b/cubbi/session.py
index b459102..ced4cb4 100644
--- a/cubbi/session.py
+++ b/cubbi/session.py
@@ -2,7 +2,9 @@
 Session storage management for Cubbi Container Tool.
 """
 
+import fcntl
 import os
+from contextlib import contextmanager
 from pathlib import Path
 from typing import Dict, Optional
 
@@ -11,6 +13,31 @@ import yaml
 DEFAULT_SESSIONS_FILE = Path.home() / ".config" / "cubbi" / "sessions.yaml"
 
 
+@contextmanager
+def _file_lock(file_path: Path):
+    """Context manager for file locking.
+
+    Args:
+        file_path: Path to the file to lock
+
+    Yields:
+        File descriptor with exclusive lock
+    """
+    # Ensure the file exists
+    file_path.parent.mkdir(parents=True, exist_ok=True)
+    if not file_path.exists():
+        file_path.touch(mode=0o600)
+
+    # Open file and acquire exclusive lock
+    fd = open(file_path, "r+")
+    try:
+        fcntl.flock(fd.fileno(), fcntl.LOCK_EX)
+        yield fd
+    finally:
+        fcntl.flock(fd.fileno(), fcntl.LOCK_UN)
+        fd.close()
+
+
 class SessionManager:
     """Manager for container sessions."""
 
@@ -42,9 +69,26 @@ class SessionManager:
         return sessions
 
     def save(self) -> None:
-        """Save the sessions to file."""
-        with open(self.sessions_path, "w") as f:
-            yaml.safe_dump(self.sessions, f)
+        """Save the sessions to file.
+
+        Note: This method acquires a file lock and merges with existing data
+        to prevent concurrent write issues.
+        """
+        with _file_lock(self.sessions_path) as fd:
+            # Reload sessions from disk to get latest state
+            fd.seek(0)
+            sessions = yaml.safe_load(fd) or {}
+
+            # Merge current in-memory sessions with disk state
+            sessions.update(self.sessions)
+
+            # Write back to file
+            fd.seek(0)
+            fd.truncate()
+            yaml.safe_dump(sessions, fd)
+
+            # Update in-memory cache
+            self.sessions = sessions
 
     def add_session(self, session_id: str, session_data: dict) -> None:
         """Add a session to storage.
@@ -53,8 +97,21 @@ class SessionManager:
             session_id: The unique session ID
             session_data: The session data (Session model dump as dict)
         """
-        self.sessions[session_id] = session_data
-        self.save()
+        with _file_lock(self.sessions_path) as fd:
+            # Reload sessions from disk to get latest state
+            fd.seek(0)
+            sessions = yaml.safe_load(fd) or {}
+
+            # Apply the modification
+            sessions[session_id] = session_data
+
+            # Write back to file
+            fd.seek(0)
+            fd.truncate()
+            yaml.safe_dump(sessions, fd)
+
+            # Update in-memory cache
+            self.sessions = sessions
 
     def get_session(self, session_id: str) -> Optional[dict]:
         """Get a session by ID.
@@ -81,6 +138,19 @@ class SessionManager:
         Args:
             session_id: The session ID to remove
         """
-        if session_id in self.sessions:
-            del self.sessions[session_id]
-            self.save()
+        with _file_lock(self.sessions_path) as fd:
+            # Reload sessions from disk to get latest state
+            fd.seek(0)
+            sessions = yaml.safe_load(fd) or {}
+
+            # Apply the modification
+            if session_id in sessions:
+                del sessions[session_id]
+
+                # Write back to file
+                fd.seek(0)
+                fd.truncate()
+                yaml.safe_dump(sessions, fd)
+
+            # Update in-memory cache
+            self.sessions = sessions