diff --git a/CHANGES b/CHANGES
index a65cbbcf0..2f54a3976 100644
--- a/CHANGES
+++ b/CHANGES
@@ -12,6 +12,8 @@ Incompatible changes
 Deprecated
 ----------
 
+* The ``app`` argument of ``sphinx.environment.BuildEnvironment`` becomes
+  required
 * ``sphinx.application.Sphinx.html_theme``
 * ``sphinx.util.docstrings.extract_metadata()``
 
diff --git a/doc/extdev/deprecated.rst b/doc/extdev/deprecated.rst
index 79081828b..f04b738de 100644
--- a/doc/extdev/deprecated.rst
+++ b/doc/extdev/deprecated.rst
@@ -22,6 +22,11 @@ The following is a list of deprecated interfaces.
      - (will be) Removed
      - Alternatives
 
+   * - The optional argument ``app`` for ``sphinx.environment.BuildEnvironment``
+     - 4.1
+     - 6.0
+     - The required argument
+
    * - ``sphinx.application.Sphinx.html_theme``
      - 4.1
      - 6.0
diff --git a/sphinx/application.py b/sphinx/application.py
index 07049ffcd..4aeca54f4 100644
--- a/sphinx/application.py
+++ b/sphinx/application.py
@@ -300,8 +300,7 @@ class Sphinx:
     def _init_env(self, freshenv: bool) -> None:
         filename = path.join(self.doctreedir, ENV_PICKLE_FILENAME)
         if freshenv or not os.path.exists(filename):
-            self.env = BuildEnvironment()
-            self.env.setup(self)
+            self.env = BuildEnvironment(self)
             self.env.find_files(self.config, self.builder)
         else:
             try:
diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index 2914a78af..15653a3fe 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -10,6 +10,7 @@
 
 import os
 import pickle
+import warnings
 from collections import defaultdict
 from copy import copy
 from datetime import datetime
@@ -22,6 +23,7 @@ from docutils.nodes import Node
 
 from sphinx import addnodes
 from sphinx.config import Config
+from sphinx.deprecation import RemovedInSphinx60Warning
 from sphinx.domains import Domain
 from sphinx.environment.adapters.toctree import TocTree
 from sphinx.errors import BuildEnvironmentError, DocumentError, ExtensionError, SphinxError
@@ -181,6 +183,9 @@ class BuildEnvironment:
         # set up environment
         if app:
             self.setup(app)
+        else:
+            warnings.warn("The 'app' argument for BuildEnvironment() becomes required now.",
+                          RemovedInSphinx60Warning, stacklevel=2)
 
     def __getstate__(self) -> Dict:
         """Obtains serializable data for pickling."""
diff --git a/sphinx/ext/autosummary/__init__.py b/sphinx/ext/autosummary/__init__.py
index 0bf921f0b..32f11baff 100644
--- a/sphinx/ext/autosummary/__init__.py
+++ b/sphinx/ext/autosummary/__init__.py
@@ -80,7 +80,9 @@ from sphinx.ext.autodoc.directive import DocumenterBridge, Options
 from sphinx.ext.autodoc.importer import import_module
 from sphinx.ext.autodoc.mock import mock
 from sphinx.locale import __
+from sphinx.project import Project
 from sphinx.pycode import ModuleAnalyzer, PycodeError
+from sphinx.registry import SphinxComponentRegistry
 from sphinx.util import logging, rst
 from sphinx.util.docutils import (NullReporter, SphinxDirective, SphinxRole, new_document,
                                   switch_source_input)
@@ -168,13 +170,24 @@ def autosummary_table_visit_html(self: HTMLTranslator, node: autosummary_table)
 _app: Sphinx = None
 
 
+class FakeApplication:
+    def __init__(self):
+        self.doctreedir = None
+        self.events = None
+        self.extensions = {}
+        self.srcdir = None
+        self.config = Config()
+        self.project = Project(None, None)
+        self.registry = SphinxComponentRegistry()
+
+
 class FakeDirective(DocumenterBridge):
     def __init__(self) -> None:
         settings = Struct(tab_width=8)
         document = Struct(settings=settings)
-        env = BuildEnvironment()
-        env.config = Config()
-        env.config.add('autodoc_class_signature', 'mixed', True, None)
+        app = FakeApplication()
+        app.config.add('autodoc_class_signature', 'mixed', True, None)
+        env = BuildEnvironment(app)  # type: ignore
         state = Struct(document=document)
         super().__init__(env, None, Options(), 0, state)