diff --git a/spanner/google/cloud/spanner/database.py b/spanner/google/cloud/spanner/database.py
index b098f7684b7c..40dcc471d1c4 100644
--- a/spanner/google/cloud/spanner/database.py
+++ b/spanner/google/cloud/spanner/database.py
@@ -15,6 +15,7 @@
 """User friendly container for Cloud Spanner Database."""
 
 import re
+import threading
 
 import google.auth.credentials
 from google.gax.errors import GaxError
@@ -79,6 +80,7 @@ def __init__(self, database_id, instance, ddl_statements=(), pool=None):
         self.database_id = database_id
         self._instance = instance
         self._ddl_statements = _check_ddl_statements(ddl_statements)
+        self._local = threading.local()
 
         if pool is None:
             pool = BurstyPool()
@@ -332,8 +334,20 @@ def run_in_transaction(self, func, *args, **kw):
         :rtype: :class:`datetime.datetime`
         :returns: timestamp of committed transaction
         """
-        with SessionCheckout(self._pool) as session:
-            return session.run_in_transaction(func, *args, **kw)
+        # Sanity check: Is there a transaction already running?
+        # If there is, then raise a red flag. Otherwise, mark that this one
+        # is running.
+        if getattr(self._local, 'transaction_running', False):
+            raise RuntimeError('Spanner does not support nested transactions.')
+        self._local.transaction_running = True
+
+        # Check out a session and run the function in a transaction; once
+        # done, flip the sanity check bit back.
+        try:
+            with SessionCheckout(self._pool) as session:
+                return session.run_in_transaction(func, *args, **kw)
+        finally:
+            self._local.transaction_running = False
 
     def batch(self):
         """Return an object which wraps a batch.
diff --git a/spanner/tests/unit/test_database.py b/spanner/tests/unit/test_database.py
index c1218599b3b3..c812176499dd 100644
--- a/spanner/tests/unit/test_database.py
+++ b/spanner/tests/unit/test_database.py
@@ -223,7 +223,7 @@ def __init__(self, scopes=(), source=None):
                 self._scopes = scopes
                 self._source = source
 
-            def requires_scopes(self): # pragma: NO COVER
+            def requires_scopes(self):  # pragma: NO COVER
                 return True
 
             def with_scopes(self, scopes):
@@ -663,6 +663,29 @@ def test_run_in_transaction_w_args(self):
         self.assertEqual(session._retried,
                          (_unit_of_work, (SINCE,), {'until': UNTIL}))
 
+    def test_run_in_transaction_nested(self):
+        from datetime import datetime
+
+        # Perform the various setup tasks.
+        instance = _Instance(self.INSTANCE_NAME, client=_Client())
+        pool = _Pool()
+        session = _Session(run_transaction_function=True)
+        session._committed = datetime.now()
+        pool.put(session)
+        database = self._make_one(self.DATABASE_ID, instance, pool=pool)
+
+        # Define the inner function.
+        inner = mock.Mock(spec=())
+
+        # Define the nested transaction.
+        def nested_unit_of_work():
+            return database.run_in_transaction(inner)
+
+        # Attempting to run this transaction should raise RuntimeError.
+        with self.assertRaises(RuntimeError):
+            database.run_in_transaction(nested_unit_of_work)
+        self.assertEqual(inner.call_count, 0)
+
     def test_batch(self):
         from google.cloud.spanner.database import BatchCheckout
 
@@ -900,11 +923,15 @@ class _Session(object):
 
     _rows = ()
 
-    def __init__(self, database=None, name=_BaseTest.SESSION_NAME):
+    def __init__(self, database=None, name=_BaseTest.SESSION_NAME,
+                 run_transaction_function=False):
         self._database = database
         self.name = name
+        self._run_transaction_function = run_transaction_function
 
     def run_in_transaction(self, func, *args, **kw):
+        if self._run_transaction_function:
+            func(*args, **kw)
         self._retried = (func, args, kw)
         return self._committed