diff --git a/Modules/sha1module.c b/Modules/sha1module.c
index 9153557fbd..86f90be1e6 100644
--- a/Modules/sha1module.c
+++ b/Modules/sha1module.c
@@ -54,6 +54,7 @@ struct sha1_state {
 typedef struct {
     PyObject_HEAD
 
+    _PyMutex mutex;
     struct sha1_state hash_state;
 } SHA1object;
 
@@ -314,6 +315,10 @@ static SHA1object *
 newSHA1object(SHA1State *st)
 {
     SHA1object *sha = (SHA1object *)PyObject_GC_New(SHA1object, st->sha1_type);
+    if (!sha) {
+        return NULL;
+    }
+    memset(&sha->mutex, 0, sizeof(sha->mutex));
     PyObject_GC_Track(sha);
     return sha;
 }
@@ -357,7 +362,9 @@ SHA1Type_copy_impl(SHA1object *self, PyTypeObject *cls)
     if ((newobj = newSHA1object(st)) == NULL)
         return NULL;
 
+    _PyMutex_lock(&self->mutex);
     newobj->hash_state = self->hash_state;
+    _PyMutex_unlock(&self->mutex);
     return (PyObject *)newobj;
 }
 
@@ -374,7 +381,9 @@ SHA1Type_digest_impl(SHA1object *self)
     unsigned char digest[SHA1_DIGESTSIZE];
     struct sha1_state temp;
 
+    _PyMutex_lock(&self->mutex);
     temp = self->hash_state;
+    _PyMutex_unlock(&self->mutex);
     sha1_done(&temp, digest);
     return PyBytes_FromStringAndSize((const char *)digest, SHA1_DIGESTSIZE);
 }
@@ -393,7 +402,9 @@ SHA1Type_hexdigest_impl(SHA1object *self)
     struct sha1_state temp;
 
     /* Get the raw (binary) digest value */
+    _PyMutex_lock(&self->mutex);
     temp = self->hash_state;
+    _PyMutex_unlock(&self->mutex);
     sha1_done(&temp, digest);
 
     return _Py_strhex((const char *)digest, SHA1_DIGESTSIZE);
@@ -416,7 +427,9 @@ SHA1Type_update(SHA1object *self, PyObject *obj)
 
     GET_BUFFER_VIEW_OR_ERROUT(obj, &buf);
 
+    _PyMutex_lock(&self->mutex);
     sha1_process(&self->hash_state, buf.buf, buf.len);
+    _PyMutex_unlock(&self->mutex);
 
     PyBuffer_Release(&buf);
     Py_RETURN_NONE;