diff --git a/news/560.feature.rst b/news/560.feature.rst
new file mode 100644
index 0000000000..7ce4615145
--- /dev/null
+++ b/news/560.feature.rst
@@ -0,0 +1 @@
+Expose the main thread id in the FileReader's metadata attribute.
diff --git a/src/memray/_memray.pyx b/src/memray/_memray.pyx
index 0423215e9b..9db72791b6 100644
--- a/src/memray/_memray.pyx
+++ b/src/memray/_memray.pyx
@@ -754,6 +754,7 @@ cdef _create_metadata(header, peak_memory):
         peak_memory=peak_memory,
         command_line=header["command_line"],
         pid=header["pid"],
+        main_thread_id=header["main_tid"],
         python_allocator=allocator_id_to_name[header["python_allocator"]],
         has_native_traces=header["native_traces"],
         trace_python_allocators=header["trace_python_allocators"],
diff --git a/src/memray/_metadata.py b/src/memray/_metadata.py
index a44caa9718..5d5913b498 100644
--- a/src/memray/_metadata.py
+++ b/src/memray/_metadata.py
@@ -13,6 +13,7 @@ class Metadata:
     peak_memory: int
     command_line: str
     pid: int
+    main_thread_id: int
     python_allocator: str
     has_native_traces: bool
     trace_python_allocators: bool
diff --git a/tests/unit/test_reader.py b/tests/unit/test_reader.py
index f94bba82a8..4ac0a12e0f 100644
--- a/tests/unit/test_reader.py
+++ b/tests/unit/test_reader.py
@@ -1,4 +1,5 @@
 import os
+import threading
 
 import pytest
 
@@ -79,3 +80,28 @@ def test_read_pid(tmp_path):
 
     # THEN
     assert FileReader(output).metadata.pid == os.getpid()
+
+
+def test_read_tid(tmp_path):
+    # GIVEN
+    output = tmp_path / "test.bin"
+    allocator = MemoryAllocator()
+
+    def func():
+        allocator.valloc(1024)
+
+    # WHEN
+    t = threading.Thread(target=func)
+    with Tracker(output):
+        func()
+        t.start()
+        t.join()
+
+    # THEN
+    reader = FileReader(output)
+    main_tid = reader.metadata.main_thread_id
+    all_allocations = reader.get_allocation_records()
+    all_tids = tuple(allocation.tid for allocation in all_allocations)
+    assert main_tid in set(all_tids)
+    # The main thread should be the first one in the list
+    assert main_tid == all_tids[0]
diff --git a/tests/unit/test_stats_reporter.py b/tests/unit/test_stats_reporter.py
index e6883601b6..193f5e4b2b 100644
--- a/tests/unit/test_stats_reporter.py
+++ b/tests/unit/test_stats_reporter.py
@@ -99,6 +99,7 @@ def fake_stats():
             has_native_traces=False,
             trace_python_allocators=True,
             file_format=FileFormat.ALL_ALLOCATIONS,
+            main_thread_id=0x1,
         ),
         total_num_allocations=20,
         total_memory_allocated=sum(mem_allocation_list),
@@ -438,6 +439,7 @@ def test_stats_output_json(fake_stats, tmp_path):
             "has_native_traces": False,
             "trace_python_allocators": True,
             "file_format": 0,
+            "main_thread_id": 0x1,
         },
     }
     actual = json.loads(output_file.read_text())