diff --git a/openhands-sdk/openhands/sdk/conversation/event_store.py b/openhands-sdk/openhands/sdk/conversation/event_store.py
index 677f468059..cec748b65d 100644
--- a/openhands-sdk/openhands/sdk/conversation/event_store.py
+++ b/openhands-sdk/openhands/sdk/conversation/event_store.py
@@ -16,17 +16,34 @@
 
 logger = get_logger(__name__)
 
+LOCK_FILE_NAME = ".eventlog.lock"
+LOCK_TIMEOUT_SECONDS = 30
+
 
 class EventLog(EventsListBase):
+    """Persistent event log with locking for concurrent writes.
+
+    This class provides thread-safe and process-safe event storage using
+    the FileStore's locking mechanism. Events are persisted to disk and
+    can be accessed by index or event ID.
+
+    Note:
+        For LocalFileStore, file locking via flock() does NOT work reliably
+        on NFS mounts or network filesystems. Users deploying with shared
+        storage should use alternative coordination mechanisms.
+    """
+
     _fs: FileStore
     _dir: str
     _length: int
+    _lock_path: str
 
     def __init__(self, fs: FileStore, dir_path: str = EVENTS_DIR) -> None:
         self._fs = fs
         self._dir = dir_path
         self._id_to_idx: dict[EventID, int] = {}
         self._idx_to_id: dict[int, EventID] = {}
+        self._lock_path = f"{dir_path}/{LOCK_FILE_NAME}"
         self._length = self._scan_and_build_index()
 
     def get_index(self, event_id: EventID) -> int:
@@ -54,7 +71,6 @@ def __getitem__(self, idx: SupportsIndex | slice) -> Event | list[Event]:
         if isinstance(idx, slice):
             start, stop, step = idx.indices(self._length)
             return [self._get_single_item(i) for i in range(start, stop, step)]
-        # idx is int-like (SupportsIndex)
         return self._get_single_item(idx)
 
     def _get_single_item(self, idx: SupportsIndex) -> Event:
@@ -75,26 +91,82 @@ def __iter__(self) -> Iterator[Event]:
                 continue
             evt = Event.model_validate_json(txt)
             evt_id = evt.id
-            # only backfill mapping if missing
             if i not in self._idx_to_id:
                 self._idx_to_id[i] = evt_id
                 self._id_to_idx.setdefault(evt_id, i)
             yield evt
 
     def append(self, event: Event) -> None:
+        """Append an event with locking for thread/process safety.
+
+        Raises:
+            TimeoutError: If the lock cannot be acquired within LOCK_TIMEOUT_SECONDS.
+            ValueError: If an event with the same ID already exists.
+        """
         evt_id = event.id
-        # Check for duplicate ID
-        if evt_id in self._id_to_idx:
-            existing_idx = self._id_to_idx[evt_id]
-            raise ValueError(
-                f"Event with ID '{evt_id}' already exists at index {existing_idx}"
+
+        try:
+            with self._fs.lock(self._lock_path, timeout=LOCK_TIMEOUT_SECONDS):
+                # Sync with disk in case another process wrote while we waited
+                disk_length = self._count_events_on_disk()
+                if disk_length > self._length:
+                    self._sync_from_disk(disk_length)
+
+                if evt_id in self._id_to_idx:
+                    existing_idx = self._id_to_idx[evt_id]
+                    raise ValueError(
+                        f"Event with ID '{evt_id}' already exists at index "
+                        f"{existing_idx}"
+                    )
+
+                target_path = self._path(self._length, event_id=evt_id)
+                self._fs.write(target_path, event.model_dump_json(exclude_none=True))
+                self._idx_to_id[self._length] = evt_id
+                self._id_to_idx[evt_id] = self._length
+                self._length += 1
+        except TimeoutError:
+            logger.error(
+                f"Failed to acquire EventLog lock within {LOCK_TIMEOUT_SECONDS}s "
+                f"for event {evt_id}"
             )
+            raise
 
-        path = self._path(self._length, event_id=evt_id)
-        self._fs.write(path, event.model_dump_json(exclude_none=True))
-        self._idx_to_id[self._length] = evt_id
-        self._id_to_idx[evt_id] = self._length
-        self._length += 1
+    def _count_events_on_disk(self) -> int:
+        """Count event files on disk."""
+        try:
+            paths = self._fs.list(self._dir)
+        except FileNotFoundError:
+            # Directory doesn't exist yet - expected for new event logs
+            return 0
+        except Exception as e:
+            logger.warning("Error listing event directory %s: %s", self._dir, e)
+            return 0
+        return sum(
+            1
+            for p in paths
+            if p.rsplit("/", 1)[-1].startswith("event-") and p.endswith(".json")
+        )
+
+    def _sync_from_disk(self, disk_length: int) -> None:
+        """Sync state for events written by other processes.
+
+        Preserves existing index mappings and only scans new events.
+        """
+        # Preserve existing mappings
+        existing_idx_to_id = dict(self._idx_to_id)
+
+        # Re-scan to pick up new events
+        scanned_length = self._scan_and_build_index()
+
+        # Restore any mappings that were lost (e.g., for non-UUID event IDs)
+        for idx, evt_id in existing_idx_to_id.items():
+            if idx not in self._idx_to_id:
+                self._idx_to_id[idx] = evt_id
+            if evt_id not in self._id_to_idx:
+                self._id_to_idx[evt_id] = idx
+
+        # Use the higher of scanned length or disk_length
+        self._length = max(scanned_length, disk_length)
 
     def __len__(self) -> int:
         return self._length
diff --git a/openhands-sdk/openhands/sdk/io/base.py b/openhands-sdk/openhands/sdk/io/base.py
index 2f93b40b4f..ce0d50afee 100644
--- a/openhands-sdk/openhands/sdk/io/base.py
+++ b/openhands-sdk/openhands/sdk/io/base.py
@@ -1,4 +1,6 @@
 from abc import ABC, abstractmethod
+from collections.abc import Iterator
+from contextlib import contextmanager
 
 
 class FileStore(ABC):
@@ -6,6 +8,9 @@ class FileStore(ABC):
 
     This class defines the interface for file storage backends that can
     handle basic file operations like reading, writing, listing, and deleting files.
+
+    Implementations should provide a locking mechanism via the `lock()` context
+    manager for thread/process-safe operations.
     """
 
     @abstractmethod
@@ -46,3 +51,50 @@ def delete(self, path: str) -> None:
         Args:
             path: The file or directory path to delete.
         """
+
+    @abstractmethod
+    def exists(self, path: str) -> bool:
+        """Check if a file or directory exists at the specified path.
+
+        Args:
+            path: The file or directory path to check.
+
+        Returns:
+            True if the path exists, False otherwise.
+        """
+
+    @abstractmethod
+    def get_absolute_path(self, path: str) -> str:
+        """Get the absolute filesystem path for a given relative path.
+
+        Args:
+            path: The relative path within the file store.
+
+        Returns:
+            The absolute path on the filesystem.
+        """
+
+    @abstractmethod
+    @contextmanager
+    def lock(self, path: str, timeout: float = 30.0) -> Iterator[None]:
+        """Acquire an exclusive lock for the given path.
+
+        This context manager provides thread and process-safe locking.
+        Implementations may use file-based locking, threading locks, or
+        other mechanisms as appropriate.
+
+        Args:
+            path: The path to lock (used to identify the lock).
+            timeout: Maximum seconds to wait for lock acquisition.
+
+        Yields:
+            None when lock is acquired.
+
+        Raises:
+            TimeoutError: If lock cannot be acquired within timeout.
+
+        Note:
+            File-based locking (flock) does NOT work reliably on NFS mounts
+            or network filesystems.
+        """
+        yield  # pragma: no cover
diff --git a/openhands-sdk/openhands/sdk/io/local.py b/openhands-sdk/openhands/sdk/io/local.py
index 86dfd05ff6..3a656d43ec 100644
--- a/openhands-sdk/openhands/sdk/io/local.py
+++ b/openhands-sdk/openhands/sdk/io/local.py
@@ -1,5 +1,9 @@
 import os
 import shutil
+from collections.abc import Iterator
+from contextlib import contextmanager
+
+from filelock import FileLock, Timeout
 
 from openhands.sdk.io.cache import MemoryLRUCache
 from openhands.sdk.logger import get_logger
@@ -117,3 +121,24 @@ def delete(self, path: str) -> None:
 
         except Exception as e:
             logger.error(f"Error clearing local file store: {str(e)}")
+
+    def exists(self, path: str) -> bool:
+        """Check if a file or directory exists."""
+        return os.path.exists(self.get_full_path(path))
+
+    def get_absolute_path(self, path: str) -> str:
+        """Get absolute filesystem path."""
+        return self.get_full_path(path)
+
+    @contextmanager
+    def lock(self, path: str, timeout: float = 30.0) -> Iterator[None]:
+        """Acquire file-based lock using flock."""
+        lock_path = self.get_full_path(path)
+        os.makedirs(os.path.dirname(lock_path), exist_ok=True)
+        file_lock = FileLock(lock_path)
+        try:
+            with file_lock.acquire(timeout=timeout):
+                yield
+        except Timeout:
+            logger.error(f"Failed to acquire lock within {timeout}s: {lock_path}")
+            raise TimeoutError(f"Lock acquisition timed out: {path}")
diff --git a/openhands-sdk/openhands/sdk/io/memory.py b/openhands-sdk/openhands/sdk/io/memory.py
index a03d0f1015..ed7c71cc0c 100644
--- a/openhands-sdk/openhands/sdk/io/memory.py
+++ b/openhands-sdk/openhands/sdk/io/memory.py
@@ -1,4 +1,8 @@
 import os
+import threading
+import uuid
+from collections.abc import Iterator
+from contextlib import contextmanager
 
 from openhands.sdk.io.base import FileStore
 from openhands.sdk.logger import get_logger
@@ -9,9 +13,13 @@
 
 class InMemoryFileStore(FileStore):
     files: dict[str, str]
+    _instance_id: str
+    _lock: threading.Lock
 
     def __init__(self, files: dict[str, str] | None = None) -> None:
         self.files = {}
+        self._instance_id = uuid.uuid4().hex
+        self._lock = threading.Lock()
         if files is not None:
             self.files = files
 
@@ -51,4 +59,29 @@ def delete(self, path: str) -> None:
                 del self.files[key]
             logger.debug(f"Cleared in-memory file store: {path}")
         except Exception as e:
-            logger.error(f"Error clearing in-memory file store: {str(e)}")
+            logger.error(f"Error clearing in-memory file store: {e}")
+
+    def exists(self, path: str) -> bool:
+        """Check if a file exists."""
+        if path in self.files:
+            return True
+        return any(f.startswith(path + "/") for f in self.files)
+
+    def get_absolute_path(self, path: str) -> str:
+        """Get absolute path (uses temp dir with unique instance ID)."""
+        import tempfile
+
+        return os.path.join(
+            tempfile.gettempdir(), f"openhands_inmemory_{self._instance_id}", path
+        )
+
+    @contextmanager
+    def lock(self, path: str, timeout: float = 30.0) -> Iterator[None]:
+        """Acquire thread lock for in-memory store."""
+        acquired = self._lock.acquire(timeout=timeout)
+        if not acquired:
+            raise TimeoutError(f"Lock acquisition timed out: {path}")
+        try:
+            yield
+        finally:
+            self._lock.release()
diff --git a/openhands-sdk/pyproject.toml b/openhands-sdk/pyproject.toml
index aefabb02cd..2bffd402d1 100644
--- a/openhands-sdk/pyproject.toml
+++ b/openhands-sdk/pyproject.toml
@@ -7,6 +7,7 @@ requires-python = ">=3.12"
 dependencies = [
     "deprecation>=2.1.0",
     "fastmcp>=2.11.3",
+    "filelock>=3.20.1",
     "httpx>=0.27.0",
     "litellm>=1.80.10",
     "pydantic>=2.12.5",
@@ -14,7 +15,7 @@ dependencies = [
     "python-json-logger>=3.3.0",
     "tenacity>=9.1.2",
     "websockets>=12",
-    "lmnr>=0.7.24"
+    "lmnr>=0.7.24",
 ]
 
 [project.optional-dependencies]
diff --git a/tests/sdk/conversation/test_event_store.py b/tests/sdk/conversation/test_event_store.py
index 2f37b6d994..f0d988262b 100644
--- a/tests/sdk/conversation/test_event_store.py
+++ b/tests/sdk/conversation/test_event_store.py
@@ -243,13 +243,14 @@ def test_event_log_index_gaps_detection():
 
 def test_event_log_file_store_exceptions():
     """Test handling of file store exceptions."""
-    # Mock file store that raises exceptions
+    import tempfile
+
     mock_fs = Mock()
     mock_fs.list.side_effect = Exception("File system error")
-
-    # Should handle gracefully
-    log = EventLog(mock_fs)
-    assert len(log) == 0
+    with tempfile.TemporaryDirectory() as temp_dir:
+        mock_fs.get_absolute_path.return_value = f"{temp_dir}/.eventlog.lock"
+        log = EventLog(mock_fs)
+        assert len(log) == 0
 
 
 def test_event_log_iteration_with_missing_files():
@@ -343,3 +344,61 @@ def test_event_log_large_index_formatting():
 
     assert log.get_index("large-index-event") == 99999
     assert log.get_id(99999) == "large-index-event"
+
+
+def test_event_log_concurrent_append_thread_safety():
+    """Test concurrent appends from multiple threads."""
+    import tempfile
+    import threading
+
+    from openhands.sdk.io.local import LocalFileStore
+
+    with tempfile.TemporaryDirectory() as temp_dir:
+        fs = LocalFileStore(temp_dir)
+        log = EventLog(fs)
+        errors: list[Exception] = []
+        lock = threading.Lock()
+
+        def append_events(thread_id: int, num_events: int):
+            for i in range(num_events):
+                try:
+                    event = create_test_event(
+                        f"t{thread_id}-e{i}", f"Thread {thread_id}"
+                    )
+                    log.append(event)
+                except Exception as e:
+                    with lock:
+                        errors.append(e)
+
+        threads = []
+        for t_id in range(5):
+            t = threading.Thread(target=append_events, args=(t_id, 10))
+            threads.append(t)
+            t.start()
+
+        for t in threads:
+            t.join()
+
+        assert len(errors) == 0, f"Errors: {errors}"
+        assert len(log) == 50
+
+
+def test_event_log_concurrent_writes_serialized():
+    """Test two EventLog instances serialize writes correctly."""
+    import tempfile
+
+    from openhands.sdk.io.local import LocalFileStore
+
+    with tempfile.TemporaryDirectory() as temp_dir:
+        fs = LocalFileStore(temp_dir)
+        log1 = EventLog(fs)
+        log2 = EventLog(fs)
+
+        log1.append(create_test_event("event-1", "First"))
+        log2.append(create_test_event("event-2", "Second"))
+
+        assert log1._length == 1
+        assert log2._length == 2
+
+        files = [f for f in fs.list("events") if not f.endswith(".lock")]
+        assert len(files) == 2
diff --git a/uv.lock b/uv.lock
index 2e156324d8..7feb970218 100644
--- a/uv.lock
+++ b/uv.lock
@@ -2149,6 +2149,7 @@ source = { editable = "openhands-sdk" }
 dependencies = [
     { name = "deprecation" },
     { name = "fastmcp" },
+    { name = "filelock" },
     { name = "httpx" },
     { name = "litellm" },
     { name = "lmnr" },
@@ -2169,6 +2170,7 @@ requires-dist = [
     { name = "boto3", marker = "extra == 'boto3'", specifier = ">=1.35.0" },
     { name = "deprecation", specifier = ">=2.1.0" },
     { name = "fastmcp", specifier = ">=2.11.3" },
+    { name = "filelock", specifier = ">=3.15.0" },
     { name = "httpx", specifier = ">=0.27.0" },
     { name = "litellm", specifier = ">=1.80.10" },
     { name = "lmnr", specifier = ">=0.7.24" },