diff --git a/CHANGES b/CHANGES
index 99e5cdaf9b..a5f5acd565 100644
--- a/CHANGES
+++ b/CHANGES
@@ -7,9 +7,14 @@ Dependencies
 Incompatible changes
 --------------------
 
+* #8105: autodoc: the signature of class constructor will be shown for decorated
+  classes, not a signature of decorator
+
 Deprecated
 ----------
 
+* The ``follow_wrapped`` argument of ``sphinx.util.inspect.signature()``
+
 Features added
 --------------
 
@@ -20,6 +25,9 @@ Features added
 Bugs fixed
 ----------
 
+* #8105: autodoc: the signature of class constructor is incorrect if the class
+  is decorated
+
 Testing
 --------
 
diff --git a/doc/extdev/deprecated.rst b/doc/extdev/deprecated.rst
index 2bb8aebfd6..797648a1ac 100644
--- a/doc/extdev/deprecated.rst
+++ b/doc/extdev/deprecated.rst
@@ -26,6 +26,12 @@ The following is a list of deprecated interfaces.
      - (will be) Removed
      - Alternatives
 
+
+   * - The ``follow_wrapped`` argument of ``sphinx.util.inspect.signature()``
+     - 3.4
+     - 5.0
+     - N/A
+
    * - ``sphinx.builders.latex.LaTeXBuilder.usepackages``
      - 3.3
      - 5.0
diff --git a/sphinx/ext/autodoc/__init__.py b/sphinx/ext/autodoc/__init__.py
index 7343d41b6a..d1e9726b7b 100644
--- a/sphinx/ext/autodoc/__init__.py
+++ b/sphinx/ext/autodoc/__init__.py
@@ -1213,7 +1213,7 @@ def format_args(self, **kwargs: Any) -> str:
 
         try:
             self.env.app.emit('autodoc-before-process-signature', self.object, False)
-            sig = inspect.signature(self.object, follow_wrapped=True,
+            sig = inspect.signature(self.object,
                                     type_aliases=self.env.config.autodoc_type_aliases)
             args = stringify_signature(sig, **kwargs)
         except TypeError as exc:
@@ -1848,7 +1848,6 @@ def format_args(self, **kwargs: Any) -> str:
                 else:
                     self.env.app.emit('autodoc-before-process-signature', self.object, True)
                     sig = inspect.signature(self.object, bound_method=True,
-                                            follow_wrapped=True,
                                             type_aliases=self.env.config.autodoc_type_aliases)
                 args = stringify_signature(sig, **kwargs)
         except TypeError as exc:
diff --git a/sphinx/util/inspect.py b/sphinx/util/inspect.py
index f2cd8070bb..4f8e15c046 100644
--- a/sphinx/util/inspect.py
+++ b/sphinx/util/inspect.py
@@ -439,14 +439,20 @@ def _should_unwrap(subject: Callable) -> bool:
     return False
 
 
-def signature(subject: Callable, bound_method: bool = False, follow_wrapped: bool = False,
+def signature(subject: Callable, bound_method: bool = False, follow_wrapped: bool = None,
               type_aliases: Dict = {}) -> inspect.Signature:
     """Return a Signature object for the given *subject*.
 
     :param bound_method: Specify *subject* is a bound method or not
     :param follow_wrapped: Same as ``inspect.signature()``.
-                           Defaults to ``False`` (get a signature of *subject*).
     """
+
+    if follow_wrapped is None:
+        follow_wrapped = True
+    else:
+        warnings.warn('The follow_wrapped argument of sphinx.util.inspect.signature() is '
+                      'deprecated', RemovedInSphinx50Warning, stacklevel=2)
+
     try:
         try:
             if _should_unwrap(subject):
diff --git a/tests/roots/test-ext-autodoc/target/decorator.py b/tests/roots/test-ext-autodoc/target/decorator.py
index 61398b324e..faad3fff95 100644
--- a/tests/roots/test-ext-autodoc/target/decorator.py
+++ b/tests/roots/test-ext-autodoc/target/decorator.py
@@ -29,3 +29,25 @@ class Bar:
     @deco1
     def meth(self, name=None, age=None):
         pass
+
+
+class Baz:
+    @deco1
+    def __init__(self, name=None, age=None):
+        pass
+
+
+class Qux:
+    @deco1
+    def __new__(self, name=None, age=None):
+        pass
+
+
+class _Metaclass(type):
+    @deco1
+    def __call__(self, name=None, age=None):
+        pass
+
+
+class Quux(metaclass=_Metaclass):
+    pass
diff --git a/tests/test_ext_autodoc_autoclass.py b/tests/test_ext_autodoc_autoclass.py
new file mode 100644
index 0000000000..66724d5a3d
--- /dev/null
+++ b/tests/test_ext_autodoc_autoclass.py
@@ -0,0 +1,41 @@
+"""
+    test_ext_autodoc_autoclass
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    Test the autodoc extension.  This tests mainly the Documenters; the auto
+    directives are tested in a test source file translated by test_build.
+
+    :copyright: Copyright 2007-2020 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')
+def test_decorators(app):
+    actual = do_autodoc(app, 'class', 'target.decorator.Baz')
+    assert list(actual) == [
+        '',
+        '.. py:class:: Baz(name=None, age=None)',
+        '   :module: target.decorator',
+        '',
+    ]
+
+    actual = do_autodoc(app, 'class', 'target.decorator.Qux')
+    assert list(actual) == [
+        '',
+        '.. py:class:: Qux(name=None, age=None)',
+        '   :module: target.decorator',
+        '',
+    ]
+
+    actual = do_autodoc(app, 'class', 'target.decorator.Quux')
+    assert list(actual) == [
+        '',
+        '.. py:class:: Quux(name=None, age=None)',
+        '   :module: target.decorator',
+        '',
+    ]