diff --git a/CHANGES b/CHANGES
index 84127da87fe..96d66ab268a 100644
--- a/CHANGES
+++ b/CHANGES
@@ -35,6 +35,10 @@ Features added
   info-field-list
 * #8514: autodoc: Default values of overloaded functions are taken from actual
   implementation if they're ellipsis
+* #759: autodoc: Add a new extension ``sphinx.ext.autodoc.preserve_defaults``.
+  It preserves the default argument values of function signatures in source code
+  and keep them not evaluated for readability.  This is an experimental
+  extension and it will be integrated into autodoc core in Sphinx-4.0
 * #8619: html: kbd role generates customizable HTML tags for compound keys
 * #8634: html: Allow to change the order of JS/CSS via ``priority`` parameter
   for :meth:`Sphinx.add_js_file()` and :meth:`Sphinx.add_css_file()`
diff --git a/sphinx/ext/autodoc/preserve_defaults.py b/sphinx/ext/autodoc/preserve_defaults.py
new file mode 100644
index 00000000000..36ef3518ce5
--- /dev/null
+++ b/sphinx/ext/autodoc/preserve_defaults.py
@@ -0,0 +1,82 @@
+"""
+    sphinx.ext.autodoc.preserve_defaults
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    Preserve the default argument values of function signatures in source code
+    and keep them not evaluated for readability.
+
+    :copyright: Copyright 2007-2021 by the Sphinx team, see AUTHORS.
+    :license: BSD, see LICENSE for details.
+"""
+
+import ast
+import inspect
+from typing import Any, Dict
+
+from sphinx.application import Sphinx
+from sphinx.locale import __
+from sphinx.pycode.ast import parse as ast_parse
+from sphinx.pycode.ast import unparse as ast_unparse
+from sphinx.util import logging
+
+logger = logging.getLogger(__name__)
+
+
+class DefaultValue:
+    def __init__(self, name: str) -> None:
+        self.name = name
+
+    def __repr__(self) -> str:
+        return self.name
+
+
+def get_function_def(obj: Any) -> ast.FunctionDef:
+    """Get FunctionDef object from living object.
+    This tries to parse original code for living object and returns
+    AST node for given *obj*.
+    """
+    try:
+        source = inspect.getsource(obj)
+        if source.startswith((' ', r'\t')):
+            # subject is placed inside class or block.  To read its docstring,
+            # this adds if-block before the declaration.
+            module = ast_parse('if True:\n' + source)
+            return module.body[0].body[0]  # type: ignore
+        else:
+            module = ast_parse(source)
+            return module.body[0]  # type: ignore
+    except (OSError, TypeError):  # failed to load source code
+        return None
+
+
+def update_defvalue(app: Sphinx, obj: Any, bound_method: bool) -> None:
+    """Update defvalue info of *obj* using type_comments."""
+    try:
+        function = get_function_def(obj)
+        if function.args.defaults or function.args.kw_defaults:
+            sig = inspect.signature(obj)
+            defaults = list(function.args.defaults)
+            kw_defaults = list(function.args.kw_defaults)
+            parameters = list(sig.parameters.values())
+            for i, param in enumerate(parameters):
+                if param.default is not param.empty:
+                    if param.kind in (param.POSITIONAL_ONLY, param.POSITIONAL_OR_KEYWORD):
+                        value = DefaultValue(ast_unparse(defaults.pop(0)))
+                        parameters[i] = param.replace(default=value)
+                    else:
+                        value = DefaultValue(ast_unparse(kw_defaults.pop(0)))
+                        parameters[i] = param.replace(default=value)
+            sig = sig.replace(parameters=parameters)
+            obj.__signature__ = sig
+    except NotImplementedError as exc:  # failed to ast.unparse()
+        logger.warning(__("Failed to parse type_comment for %r: %s"), obj, exc)
+
+
+def setup(app: Sphinx) -> Dict[str, Any]:
+    app.setup_extension('sphinx.ext.autodoc')
+    app.connect('autodoc-before-process-signature', update_defvalue)
+
+    return {
+        'version': '1.0',
+        'parallel_read_safe': True
+    }
diff --git a/tests/roots/test-ext-autodoc/target/preserve_defaults.py b/tests/roots/test-ext-autodoc/target/preserve_defaults.py
new file mode 100644
index 00000000000..5b60ec38e92
--- /dev/null
+++ b/tests/roots/test-ext-autodoc/target/preserve_defaults.py
@@ -0,0 +1,20 @@
+from datetime import datetime
+from typing import Any
+
+
+CONSTANT = 'foo'
+SENTINEL = object()
+
+
+def foo(name: str = CONSTANT,
+        sentinal: Any = SENTINEL,
+        now: datetime = datetime.now()) -> None:
+    """docstring"""
+
+
+class Class:
+    """docstring"""
+
+    def meth(self, name: str = CONSTANT, sentinal: Any = SENTINEL,
+             now: datetime = datetime.now()) -> None:
+        """docstring"""
diff --git a/tests/test_ext_autodoc_preserve_defaults.py b/tests/test_ext_autodoc_preserve_defaults.py
new file mode 100644
index 00000000000..7a5a8091465
--- /dev/null
+++ b/tests/test_ext_autodoc_preserve_defaults.py
@@ -0,0 +1,45 @@
+"""
+    test_ext_autodoc_preserve_defaults
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    Test the autodoc extension.
+
+    :copyright: Copyright 2007-2021 by the Sphinx team, see AUTHORS.
+    :license: BSD, see LICENSE for details.
+"""
+
+import pytest
+
+from .test_ext_autodoc import do_autodoc
+
+
+@pytest.mark.sphinx('html', testroot='ext-autodoc',
+                    confoverrides={'extensions': ['sphinx.ext.autodoc.preserve_defaults']})
+def test_preserve_defaults(app):
+    options = {"members": None}
+    actual = do_autodoc(app, 'module', 'target.preserve_defaults', options)
+    assert list(actual) == [
+        '',
+        '.. py:module:: target.preserve_defaults',
+        '',
+        '',
+        '.. py:class:: Class',
+        '   :module: target.preserve_defaults',
+        '',
+        '   docstring',
+        '',
+        '',
+        '   .. py:method:: Class.meth(name: str = CONSTANT, sentinal: Any = SENTINEL, '
+        'now: datetime.datetime = datetime.now()) -> None',
+        '      :module: target.preserve_defaults',
+        '',
+        '      docstring',
+        '',
+        '',
+        '.. py:function:: foo(name: str = CONSTANT, sentinal: Any = SENTINEL, now: '
+        'datetime.datetime = datetime.now()) -> None',
+        '   :module: target.preserve_defaults',
+        '',
+        '   docstring',
+        '',
+    ]