diff --git a/CHANGES.rst b/CHANGES.rst
index 96e6b18da..64a80c385 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -48,6 +48,10 @@ Bugs fixed
 * #12380: LaTeX: Footnote mark sometimes indicates ``Page N`` where ``N`` is
   the current page number and the footnote does appear on that same page.
   Patch by Jean-François B.
+* #12416: :confval:`root_doc` is synchronized with :confval:`master_doc`
+  so that if either of the two values is modified, the other reflects that
+  modification. It is still recommended to use :confval:`root_doc`.
+  Patch by Bénédikt Tran.
 
 Testing
 -------
diff --git a/sphinx/builders/epub3.py b/sphinx/builders/epub3.py
index 91c76e41c..1e02787df 100644
--- a/sphinx/builders/epub3.py
+++ b/sphinx/builders/epub3.py
@@ -255,7 +255,7 @@ def convert_epub_css_files(app: Sphinx, config: Config) -> None:
                 logger.warning(__('invalid css_file: %r, ignored'), entry)
                 continue
 
-    config.epub_css_files = epub_css_files  # type: ignore[attr-defined]
+    config.epub_css_files = epub_css_files
 
 
 def setup(app: Sphinx) -> ExtensionMetadata:
diff --git a/sphinx/builders/gettext.py b/sphinx/builders/gettext.py
index fb5bd11e0..274179b98 100644
--- a/sphinx/builders/gettext.py
+++ b/sphinx/builders/gettext.py
@@ -299,9 +299,9 @@ def _gettext_compact_validator(app: Sphinx, config: Config) -> None:
     gettext_compact = config.gettext_compact
     # Convert 0/1 from the command line to ``bool`` types
     if gettext_compact == '0':
-        config.gettext_compact = False  # type: ignore[attr-defined]
+        config.gettext_compact = False
     elif gettext_compact == '1':
-        config.gettext_compact = True  # type: ignore[attr-defined]
+        config.gettext_compact = True
 
 
 def setup(app: Sphinx) -> ExtensionMetadata:
diff --git a/sphinx/builders/html/__init__.py b/sphinx/builders/html/__init__.py
index 75b0a394b..ee4474b64 100644
--- a/sphinx/builders/html/__init__.py
+++ b/sphinx/builders/html/__init__.py
@@ -1187,7 +1187,7 @@ def convert_html_css_files(app: Sphinx, config: Config) -> None:
                 logger.warning(__('invalid css_file: %r, ignored'), entry)
                 continue
 
-    config.html_css_files = html_css_files  # type: ignore[attr-defined]
+    config.html_css_files = html_css_files
 
 
 def _format_modified_time(timestamp: float) -> str:
@@ -1210,7 +1210,7 @@ def convert_html_js_files(app: Sphinx, config: Config) -> None:
                 logger.warning(__('invalid js_file: %r, ignored'), entry)
                 continue
 
-    config.html_js_files = html_js_files  # type: ignore[attr-defined]
+    config.html_js_files = html_js_files
 
 
 def setup_resource_paths(app: Sphinx, pagename: str, templatename: str,
@@ -1273,7 +1273,7 @@ def validate_html_logo(app: Sphinx, config: Config) -> None:
             not path.isfile(path.join(app.confdir, config.html_logo)) and
             not isurl(config.html_logo)):
         logger.warning(__('logo file %r does not exist'), config.html_logo)
-        config.html_logo = None  # type: ignore[attr-defined]
+        config.html_logo = None
 
 
 def validate_html_favicon(app: Sphinx, config: Config) -> None:
@@ -1282,7 +1282,7 @@ def validate_html_favicon(app: Sphinx, config: Config) -> None:
             not path.isfile(path.join(app.confdir, config.html_favicon)) and
             not isurl(config.html_favicon)):
         logger.warning(__('favicon file %r does not exist'), config.html_favicon)
-        config.html_favicon = None  # type: ignore[attr-defined]
+        config.html_favicon = None
 
 
 def error_on_html_4(_app: Sphinx, config: Config) -> None:
diff --git a/sphinx/config.py b/sphinx/config.py
index 1f4b47067..437814995 100644
--- a/sphinx/config.py
+++ b/sphinx/config.py
@@ -385,6 +385,15 @@ class Config:
             values.append(f"{opt_name}={opt_value!r}")
         return self.__class__.__qualname__ + '(' + ', '.join(values) + ')'
 
+    def __setattr__(self, key: str, value: Any) -> None:
+        # if someone is still using 'master_doc', we need to update 'root_doc'
+        if key in ('master_doc', 'root_doc'):
+            super().__setattr__('root_doc', value)
+            super().__setattr__('master_doc', value)
+            return
+
+        super().__setattr__(key, value)
+
     def __getattr__(self, name: str) -> Any:
         if name in self._options:
             # first check command-line overrides
@@ -561,10 +570,10 @@ def convert_source_suffix(app: Sphinx, config: Config) -> None:
         #
         # The default filetype is determined on later step.
         # By default, it is considered as restructuredtext.
-        config.source_suffix = {source_suffix: None}  # type: ignore[attr-defined]
+        config.source_suffix = {source_suffix: None}
     elif isinstance(source_suffix, (list, tuple)):
         # if list, considers as all of them are default filetype
-        config.source_suffix = dict.fromkeys(source_suffix, None)  # type: ignore[attr-defined]
+        config.source_suffix = dict.fromkeys(source_suffix, None)
     elif not isinstance(source_suffix, dict):
         logger.warning(__("The config value `source_suffix' expects "
                           "a string, list of strings, or dictionary. "
@@ -580,8 +589,7 @@ def convert_highlight_options(app: Sphinx, config: Config) -> None:
     options = config.highlight_options
     if options and not all(isinstance(v, dict) for v in options.values()):
         # old styled option detected because all values are not dictionary.
-        config.highlight_options = {config.highlight_language:  # type: ignore[attr-defined]
-                                    options}
+        config.highlight_options = {config.highlight_language: options}
 
 
 def init_numfig_format(app: Sphinx, config: Config) -> None:
@@ -593,7 +601,7 @@ def init_numfig_format(app: Sphinx, config: Config) -> None:
 
     # override default labels by configuration
     numfig_format.update(config.numfig_format)
-    config.numfig_format = numfig_format  # type: ignore[attr-defined]
+    config.numfig_format = numfig_format
 
 
 def correct_copyright_year(_app: Sphinx, config: Config) -> None:
@@ -713,7 +721,7 @@ def check_primary_domain(app: Sphinx, config: Config) -> None:
     primary_domain = config.primary_domain
     if primary_domain and not app.registry.has_domain(primary_domain):
         logger.warning(__('primary_domain %r not found, ignored.'), primary_domain)
-        config.primary_domain = None  # type: ignore[attr-defined]
+        config.primary_domain = None
 
 
 def check_root_doc(app: Sphinx, env: BuildEnvironment, added: set[str],
@@ -726,7 +734,7 @@ def check_root_doc(app: Sphinx, env: BuildEnvironment, added: set[str],
             'contents' in app.project.docnames):
         logger.warning(__('Since v2.0, Sphinx uses "index" as root_doc by default. '
                           'Please add "root_doc = \'contents\'" to your conf.py.'))
-        app.config.root_doc = "contents"  # type: ignore[attr-defined]
+        app.config.root_doc = "contents"
 
     return changed
 
diff --git a/sphinx/ext/autodoc/__init__.py b/sphinx/ext/autodoc/__init__.py
index 197aca25d..b3fb3e0c4 100644
--- a/sphinx/ext/autodoc/__init__.py
+++ b/sphinx/ext/autodoc/__init__.py
@@ -2721,10 +2721,10 @@ class AttributeDocumenter(GenericAliasMixin, SlotsMixin,  # type: ignore[misc]
             # a docstring from the value which descriptor returns unexpectedly.
             # ref: https://github.com/sphinx-doc/sphinx/issues/7805
             orig = self.config.autodoc_inherit_docstrings
-            self.config.autodoc_inherit_docstrings = False  # type: ignore[attr-defined]
+            self.config.autodoc_inherit_docstrings = False
             return super().get_doc()
         finally:
-            self.config.autodoc_inherit_docstrings = orig  # type: ignore[attr-defined]
+            self.config.autodoc_inherit_docstrings = orig
 
     def add_content(self, more_content: StringList | None) -> None:
         # Disable analyzing attribute comment on Documenter.add_content() to control it on
diff --git a/sphinx/ext/autosummary/generate.py b/sphinx/ext/autosummary/generate.py
index 486625639..c400d720e 100644
--- a/sphinx/ext/autosummary/generate.py
+++ b/sphinx/ext/autosummary/generate.py
@@ -740,9 +740,7 @@ def main(argv: Sequence[str] = (), /) -> None:
 
     if args.templates:
         app.config.templates_path.append(path.abspath(args.templates))
-    app.config.autosummary_ignore_module_all = (  # type: ignore[attr-defined]
-        not args.respect_module_all
-    )
+    app.config.autosummary_ignore_module_all = (not args.respect_module_all)
 
     generate_autosummary_docs(args.source_file, args.output_dir,
                               '.' + args.suffix,
diff --git a/sphinx/transforms/i18n.py b/sphinx/transforms/i18n.py
index 88b7f416e..6caf28ef6 100644
--- a/sphinx/transforms/i18n.py
+++ b/sphinx/transforms/i18n.py
@@ -64,7 +64,7 @@ def publish_msgstr(app: Sphinx, source: str, source_path: str, source_line: int,
     try:
         # clear rst_prolog temporarily
         rst_prolog = config.rst_prolog
-        config.rst_prolog = None  # type: ignore[attr-defined]
+        config.rst_prolog = None
 
         from sphinx.io import SphinxI18nReader
         reader = SphinxI18nReader()
@@ -81,7 +81,7 @@ def publish_msgstr(app: Sphinx, source: str, source_path: str, source_line: int,
             return doc[0]
         return doc
     finally:
-        config.rst_prolog = rst_prolog  # type: ignore[attr-defined]
+        config.rst_prolog = rst_prolog
 
 
 def parse_noqa(source: str) -> tuple[str, bool]:
diff --git a/tests/test_builders/test_build_linkcheck.py b/tests/test_builders/test_build_linkcheck.py
index bb91d3bf4..3d048c774 100644
--- a/tests/test_builders/test_build_linkcheck.py
+++ b/tests/test_builders/test_build_linkcheck.py
@@ -292,7 +292,7 @@ class AnchorsIgnoreForUrlHandler(BaseHTTPRequestHandler):
 @pytest.mark.sphinx('linkcheck', testroot='linkcheck-anchors-ignore-for-url', freshenv=True)
 def test_anchors_ignored_for_url(app: Sphinx) -> None:
     with serve_application(app, AnchorsIgnoreForUrlHandler) as address:
-        app.config.linkcheck_anchors_ignore_for_url = [  # type: ignore[attr-defined]
+        app.config.linkcheck_anchors_ignore_for_url = [
             f'http://{address}/ignored',  # existing page
             f'http://{address}/invalid',  # unknown page
         ]
@@ -402,7 +402,7 @@ def custom_handler(valid_credentials=(), success_criteria=lambda _: True):
 @pytest.mark.sphinx('linkcheck', testroot='linkcheck-localserver', freshenv=True)
 def test_auth_header_uses_first_match(app: Sphinx) -> None:
     with serve_application(app, custom_handler(valid_credentials=("user1", "password"))) as address:
-        app.config.linkcheck_auth = [  # type: ignore[attr-defined]
+        app.config.linkcheck_auth = [
             (r'^$', ('no', 'match')),
             (fr'^http://{re.escape(address)}/$', ('user1', 'password')),
             (r'.*local.*', ('user2', 'hunter2')),
@@ -456,7 +456,7 @@ def test_linkcheck_request_headers(app: Sphinx) -> None:
         return self.headers["Accept"] == "text/html"
 
     with serve_application(app, custom_handler(success_criteria=check_headers)) as address:
-        app.config.linkcheck_request_headers = {  # type: ignore[attr-defined]
+        app.config.linkcheck_request_headers = {
             f"http://{address}/": {"Accept": "text/html"},
             "*": {"X-Secret": "open sesami"},
         }
@@ -476,7 +476,7 @@ def test_linkcheck_request_headers_no_slash(app: Sphinx) -> None:
         return self.headers["Accept"] == "application/json"
 
     with serve_application(app, custom_handler(success_criteria=check_headers)) as address:
-        app.config.linkcheck_request_headers = {  # type: ignore[attr-defined]
+        app.config.linkcheck_request_headers = {
             f"http://{address}": {"Accept": "application/json"},
             "*": {"X-Secret": "open sesami"},
         }
@@ -579,7 +579,7 @@ def test_follows_redirects_on_GET(app, capsys, warning):
 @pytest.mark.sphinx('linkcheck', testroot='linkcheck-localserver-warn-redirects')
 def test_linkcheck_allowed_redirects(app: Sphinx, warning: StringIO) -> None:
     with serve_application(app, make_redirect_handler(support_head=False)) as address:
-        app.config.linkcheck_allowed_redirects = {f'http://{address}/.*1': '.*'}  # type: ignore[attr-defined]
+        app.config.linkcheck_allowed_redirects = {f'http://{address}/.*1': '.*'}
         compile_linkcheck_allowed_redirects(app, app.config)
         app.build()
 
diff --git a/tests/test_config/test_config.py b/tests/test_config/test_config.py
index e1cb1b093..322daa3a4 100644
--- a/tests/test_config/test_config.py
+++ b/tests/test_config/test_config.py
@@ -803,3 +803,19 @@ def test_gettext_compact_command_line_str():
 
     # regression test for #8549 (-D gettext_compact=spam)
     assert config.gettext_compact == 'spam'
+
+
+def test_root_doc_and_master_doc_are_synchronized():
+    c = Config()
+    assert c.master_doc == 'index'
+    assert c.root_doc == c.master_doc
+
+    c = Config()
+    c.master_doc = '1234'
+    assert c.master_doc == '1234'
+    assert c.root_doc == c.master_doc
+
+    c = Config()
+    c.root_doc = '1234'
+    assert c.master_doc == '1234'
+    assert c.root_doc == c.master_doc