diff --git a/core/modules/migrate/src/Plugin/migrate/process/Migration.php b/core/modules/migrate/src/Plugin/migrate/process/Migration.php
index 0fb0dc9..dcec0f6 100644
--- a/core/modules/migrate/src/Plugin/migrate/process/Migration.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/Migration.php
@@ -122,6 +122,14 @@ public function transform($value, MigrateExecutable $migrate_executable, Row $ro
       foreach (array_keys($source_ids) as $index => $source_id) {
         $values[$source_id] = $source_id_values[$migration->id()][$index];
       }
+      // If we have constants in the source then add them to the values and the
+      // process array.
+      if (isset($migration->get('source')['constants'])) {
+        $constants = $migration->get('source')['constants'];
+        $values += $constants;
+        $process += array_combine(array_keys($constants), array_keys($constants));
+      }
+
       $stub_row = new Row($values, $source_ids);
       $stub_row->stub(TRUE);
       // Do a normal migration with the stub row.