diff --git a/tools/build_api.py b/tools/build_api.py
index 54c39e59d7..b8b3c19eae 100644
--- a/tools/build_api.py
+++ b/tools/build_api.py
@@ -411,7 +411,7 @@ def _fill_header(region_list, current_region):
     return header
 
 
-def merge_region_list(region_list, destination, notify, padding=b'\xFF'):
+def merge_region_list(region_list, destination, notify, config, padding=b'\xFF'):
     """Merge the region_list into a single image
 
     Positional Arguments:
@@ -435,6 +435,13 @@ def merge_region_list(region_list, destination, notify, padding=b'\xFF'):
             notify.info("  Filling region %s with %s" % (region.name, region.filename))
             part = intelhex_offset(region.filename, offset=region.start)
             part.start_addr = None
+            # Normally, we assume that part.maxddr() can be beyond
+            # end of rom. However, if the size is restricted with config, do check.
+            if config.target.restrict_size is not None:
+                part_size = (part.maxaddr() - part.minaddr()) + 1
+                if part_size > region.size:
+                    raise ToolException("Contents of region %s does not fit"
+                                  % region.name)
             merged.merge(part)
 
     # Hex file can have gaps, so no padding needed. While other formats may
@@ -555,13 +562,13 @@ def build_project(src_paths, build_path, target, toolchain_name,
                            for r in region_list]
             res = "%s.%s" % (join(build_path, name),
                              getattr(toolchain.target, "OUTPUT_EXT", "bin"))
-            merge_region_list(region_list, res, notify)
+            merge_region_list(region_list, res, notify, toolchain.config)
             update_regions = [
                 r for r in region_list if r.name in UPDATE_WHITELIST
             ]
             if update_regions:
                 update_res = join(build_path, generate_update_filename(name, toolchain.target))
-                merge_region_list(update_regions, update_res, notify)
+                merge_region_list(update_regions, update_res, notify, toolchain.config)
                 res = (res, update_res)
             else:
                 res = (res, None)
diff --git a/tools/test/build_api/build_api_test.py b/tools/test/build_api/build_api_test.py
index 2fcfc947b8..cc397df839 100644
--- a/tools/test/build_api/build_api_test.py
+++ b/tools/test/build_api/build_api_test.py
@@ -18,11 +18,14 @@ limitations under the License.
 import unittest
 from collections import namedtuple
 from mock import patch, MagicMock
-from tools.build_api import prepare_toolchain, build_project, build_library
+from tools.build_api import prepare_toolchain, build_project, build_library, merge_region_list
 from tools.resources import Resources
 from tools.toolchains import TOOLCHAINS
 from tools.notifier.mock import MockNotifier
-
+from tools.config import Region
+from tools.utils import ToolException
+from intelhex import IntelHex
+import intelhex
 """
 Tests for build_api.py
 """
@@ -240,5 +243,36 @@ class BuildApiTests(unittest.TestCase):
         self.assertEqual(args[1]['app_config'], None,
                          "prepare_toolchain was called with an incorrect app_config")
 
+    @patch('tools.build_api.intelhex_offset')
+    @patch('tools.config')
+    def test_merge_region_no_fit(self, mock_config, mock_intelhex_offset):
+        """
+        Test that merge region fails as expected when part size overflows region size.
+        """
+        max_addr = 87444
+        # create a dummy hex file with above max_addr
+        mock_intelhex_offset.return_value = IntelHex({0:2, max_addr:0})
+
+        # create application and post-application regions and merge.
+        region_application = Region("application", 10000, 86000, True, "random.hex")
+        region_post_application = Region("postapplication", 100000, 90000, False, None)
+
+        notify = MockNotifier()
+        region_list = [region_application, region_post_application]
+        # path to store the result in, should not get used as we expect exception.
+        res = "./"
+        mock_config.target.restrict_size = 90000
+        toolexception = False
+
+        try:
+            merge_region_list(region_list, res, notify, mock_config)
+        except ToolException:
+            toolexception = True
+        except Exception as e:
+            print("%s %s" % (e.message, e.args))
+
+        self.assertTrue(toolexception, "Expected ToolException not raised")
+
+
 if __name__ == '__main__':
     unittest.main()