diff -u b/core/modules/file/migration_templates/d7_file_private.yml b/core/modules/file/migration_templates/d7_file_private.yml
--- b/core/modules/file/migration_templates/d7_file_private.yml
+++ b/core/modules/file/migration_templates/d7_file_private.yml
@@ -6,11 +6,11 @@
   plugin: d7_file
   scheme: private
   constants:
-    # source_full_path must be set by the tool configuring this migration.
+    # source_base_path must be set by the tool configuring this migration.
     # It represents the fully qualified path relative to which uris in the files
     # table are specified, and must end with a /. See source_full_path
     # configuration in this migration's process pipeline as an example.
-    source_full_path: ''
+    source_base_path: ''
 process:
   # If you are using this file to build a custom migration consider removing
   # the fid field to allow incremental migrations.
@@ -21,7 +21,7 @@
       plugin: concat
       delimiter: /
       source:
-        - constants/source_full_path
+        - constants/source_base_path
         - filepath
   uri:
     plugin: file_copy
diff -u b/core/modules/file/tests/src/Kernel/Migrate/d7/MigratePrivateFileTest.php b/core/modules/file/tests/src/Kernel/Migrate/d7/MigratePrivateFileTest.php
--- b/core/modules/file/tests/src/Kernel/Migrate/d7/MigratePrivateFileTest.php
+++ b/core/modules/file/tests/src/Kernel/Migrate/d7/MigratePrivateFileTest.php
@@ -21,14 +21,14 @@
    */
   protected function setUp() {
     parent::setUp();
-    $this->setSetting('file_private_path', 'sites/default/private');
+    $this->setSetting('file_private_path', $this->container->get('site.path') . '/private');
     $this->installEntitySchema('file');
     $fs = $this->container->get('file_system');
 
     // Ensure that the private files directory exists.
-    $fs->mkdir('private://', NULL, TRUE);
+    $fs->mkdir('private://sites/default/private/', NULL, TRUE);
     // Put test file in the source directory.
-    file_put_contents('private://Babylon5.txt', str_repeat('*', 3));
+    file_put_contents('private://sites/default/private/Babylon5.txt', str_repeat('*', 3));
 
     /** @var \Drupal\migrate\Plugin\Migration $migration */
     $migration = $this->getMigration('d7_file_private');