diff --git a/crates/common/src/lock.rs b/crates/common/src/lock.rs
index 74679bd9d8f..f1d00627e90 100644
--- a/crates/common/src/lock.rs
+++ b/crates/common/src/lock.rs
@@ -96,3 +96,18 @@ pub unsafe fn reinit_rwlock_after_fork<T: ?Sized>(rwlock: &PyRwLock<T>) {
         core::ptr::write_bytes(raw, 0, core::mem::size_of::<RawRwLock>());
     }
 }
+
+/// Reset a `PyThreadMutex` to its initial (unlocked, unowned) state after `fork()`.
+///
+/// `PyThreadMutex` is used by buffered IO objects (`BufferedReader`,
+/// `BufferedWriter`, `TextIOWrapper`). If a dead parent thread held one of
+/// these locks during `fork()`, the child would deadlock on any IO operation.
+///
+/// # Safety
+///
+/// Must only be called from the single-threaded child process immediately
+/// after `fork()`, before any other thread is created.
+#[cfg(unix)]
+pub unsafe fn reinit_thread_mutex_after_fork<T: ?Sized>(mutex: &PyThreadMutex<T>) {
+    unsafe { mutex.raw().reinit_after_fork() }
+}
diff --git a/crates/common/src/lock/thread_mutex.rs b/crates/common/src/lock/thread_mutex.rs
index 2cabf7ea4cd..5b5b89f4eb1 100644
--- a/crates/common/src/lock/thread_mutex.rs
+++ b/crates/common/src/lock/thread_mutex.rs
@@ -72,6 +72,23 @@ impl<R: RawMutex, G: GetThreadId> RawThreadMutex<R, G> {
     }
 }
 
+impl<R: RawMutex, G: GetThreadId> RawThreadMutex<R, G> {
+    /// Reset this mutex to its initial (unlocked, unowned) state after `fork()`.
+    ///
+    /// # Safety
+    ///
+    /// Must only be called from the single-threaded child process immediately
+    /// after `fork()`, before any other thread is created.
+    #[cfg(unix)]
+    pub unsafe fn reinit_after_fork(&self) {
+        self.owner.store(0, Ordering::Relaxed);
+        unsafe {
+            let mutex_ptr = &self.mutex as *const R as *mut u8;
+            core::ptr::write_bytes(mutex_ptr, 0, core::mem::size_of::<R>());
+        }
+    }
+}
+
 unsafe impl<R: RawMutex + Send, G: GetThreadId + Send> Send for RawThreadMutex<R, G> {}
 unsafe impl<R: RawMutex + Sync, G: GetThreadId + Sync> Sync for RawThreadMutex<R, G> {}
 
@@ -103,6 +120,11 @@ impl<R: RawMutex, G: GetThreadId, T> From<T> for ThreadMutex<R, G, T> {
     }
 }
 impl<R: RawMutex, G: GetThreadId, T: ?Sized> ThreadMutex<R, G, T> {
+    /// Access the underlying raw thread mutex.
+    pub fn raw(&self) -> &RawThreadMutex<R, G> {
+        &self.raw
+    }
+
     pub fn lock(&self) -> Option<ThreadMutexGuard<'_, R, G, T>> {
         if self.raw.lock() {
             Some(ThreadMutexGuard {
diff --git a/crates/vm/src/stdlib/io.rs b/crates/vm/src/stdlib/io.rs
index f75a5dd4014..945042bc9e4 100644
--- a/crates/vm/src/stdlib/io.rs
+++ b/crates/vm/src/stdlib/io.rs
@@ -2,6 +2,8 @@
  * I/O core tools.
  */
 pub(crate) use _io::module_def;
+#[cfg(all(unix, feature = "threading"))]
+pub(crate) use _io::reinit_std_streams_after_fork;
 
 cfg_if::cfg_if! {
     if #[cfg(any(not(target_arch = "wasm32"), target_os = "wasi"))] {
@@ -4985,6 +4987,61 @@ mod _io {
         }
     }
 
+    /// Reinit per-object IO buffer locks on std streams after `fork()`.
+    ///
+    /// # Safety
+    ///
+    /// Must only be called from the single-threaded child process immediately
+    /// after `fork()`, before any other thread is created.
+    #[cfg(all(unix, feature = "threading"))]
+    pub unsafe fn reinit_std_streams_after_fork(vm: &VirtualMachine) {
+        for name in ["stdin", "stdout", "stderr"] {
+            let Ok(stream) = vm.sys_module.get_attr(name, vm) else {
+                continue;
+            };
+            reinit_io_locks(&stream);
+        }
+    }
+
+    #[cfg(all(unix, feature = "threading"))]
+    fn reinit_io_locks(obj: &PyObject) {
+        use crate::common::lock::reinit_thread_mutex_after_fork;
+
+        if let Some(tio) = obj.downcast_ref::<TextIOWrapper>() {
+            unsafe { reinit_thread_mutex_after_fork(&tio.data) };
+            if let Some(guard) = tio.data.lock() {
+                if let Some(ref data) = *guard {
+                    if let Some(ref decoder) = data.decoder {
+                        reinit_io_locks(decoder);
+                    }
+                    reinit_io_locks(&data.buffer);
+                }
+            }
+            return;
+        }
+        if let Some(nl) = obj.downcast_ref::<IncrementalNewlineDecoder>() {
+            unsafe { reinit_thread_mutex_after_fork(&nl.data) };
+            return;
+        }
+        if let Some(br) = obj.downcast_ref::<BufferedReader>() {
+            unsafe { reinit_thread_mutex_after_fork(&br.data) };
+            return;
+        }
+        if let Some(bw) = obj.downcast_ref::<BufferedWriter>() {
+            unsafe { reinit_thread_mutex_after_fork(&bw.data) };
+            return;
+        }
+        if let Some(brw) = obj.downcast_ref::<BufferedRandom>() {
+            unsafe { reinit_thread_mutex_after_fork(&brw.data) };
+            return;
+        }
+        if let Some(brw) = obj.downcast_ref::<BufferedRWPair>() {
+            unsafe { reinit_thread_mutex_after_fork(&brw.read.data) };
+            unsafe { reinit_thread_mutex_after_fork(&brw.write.data) };
+            return;
+        }
+    }
+
     pub fn io_open(
         file: PyObjectRef,
         mode: Option<&str>,
diff --git a/crates/vm/src/stdlib/posix.rs b/crates/vm/src/stdlib/posix.rs
index febb3af9b0d..1adfb8cfe5a 100644
--- a/crates/vm/src/stdlib/posix.rs
+++ b/crates/vm/src/stdlib/posix.rs
@@ -719,6 +719,14 @@ pub mod module {
         #[cfg(feature = "threading")]
         reinit_locks_after_fork(vm);
 
+        // Reinit per-object IO buffer locks on std streams.
+        // BufferedReader/Writer/TextIOWrapper use PyThreadMutex which can be
+        // held by dead parent threads, causing deadlocks on any IO in the child.
+        #[cfg(feature = "threading")]
+        unsafe {
+            crate::stdlib::io::reinit_std_streams_after_fork(vm)
+        };
+
         // Phase 2: Reset low-level atomic state (no locks needed).
         crate::signal::clear_after_fork();
         crate::stdlib::signal::_signal::clear_wakeup_fd_after_fork();