diff --git a/CHANGES b/CHANGES
index e95c6a132..8bac61c1a 100644
--- a/CHANGES
+++ b/CHANGES
@@ -54,6 +54,7 @@ Deprecated
 * The ``language`` argument of ``sphinx.util.i18n:format_date()`` becomes
   required
 * ``sphinx.builders.html.html5_ready``
+* ``sphinx.io.read_doc()``
 * ``sphinx.util.docutils.__version_info__``
 * ``sphinx.util.docutils.is_html5_writer_available()``
 * ``sphinx.writers.latex.LaTeXWriter.docclasses``
diff --git a/doc/extdev/deprecated.rst b/doc/extdev/deprecated.rst
index 5a03f821a..98bd463a9 100644
--- a/doc/extdev/deprecated.rst
+++ b/doc/extdev/deprecated.rst
@@ -47,6 +47,11 @@ The following is a list of deprecated interfaces.
      - 7.0
      - N/A
 
+   * - ``sphinx.io.read_doc()``
+     - 5.0
+     - 7.0
+     - ``sphinx.builders.Builder.read_doc()``
+
    * - ``sphinx.util.docutils.__version_info__``
      - 5.0
      - 7.0
diff --git a/sphinx/io.py b/sphinx/io.py
index b4cec7d3e..5ab7b2b63 100644
--- a/sphinx/io.py
+++ b/sphinx/io.py
@@ -1,5 +1,6 @@
 """Input/Output files"""
-
+import codecs
+import warnings
 from typing import TYPE_CHECKING, Any, List, Type
 
 from docutils import nodes
@@ -7,19 +8,21 @@ from docutils.core import Publisher
 from docutils.frontend import Values
 from docutils.io import FileInput, Input, NullOutput
 from docutils.parsers import Parser
+from docutils.parsers.rst import Parser as RSTParser
 from docutils.readers import standalone
 from docutils.transforms import Transform
 from docutils.transforms.references import DanglingReferences
 from docutils.writers import UnfilteredWriter
 
 from sphinx import addnodes
+from sphinx.deprecation import RemovedInSphinx70Warning
 from sphinx.environment import BuildEnvironment
 from sphinx.transforms import (AutoIndexUpgrader, DoctreeReadEvent, FigureAligner,
                                SphinxTransformer)
 from sphinx.transforms.i18n import (Locale, PreserveTranslatableMessages,
                                     RemoveTranslatableInline)
 from sphinx.transforms.references import SphinxDomains
-from sphinx.util import logging
+from sphinx.util import UnicodeDecodeErrorHandler, get_filetype, logging
 from sphinx.util.docutils import LoggingReporter
 from sphinx.versioning import UIDTransform
 
@@ -152,6 +155,38 @@ class SphinxFileInput(FileInput):
         super().__init__(*args, **kwargs)
 
 
+def read_doc(app: "Sphinx", env: BuildEnvironment, filename: str) -> nodes.document:
+    """Parse a document and convert to doctree."""
+    warnings.warn('sphinx.io.read_doc() is deprecated.',
+                  RemovedInSphinx70Warning, stacklevel=2)
+
+    # set up error_handler for the target document
+    error_handler = UnicodeDecodeErrorHandler(env.docname)
+    codecs.register_error('sphinx', error_handler)  # type: ignore
+
+    reader = SphinxStandaloneReader()
+    reader.setup(app)
+    filetype = get_filetype(app.config.source_suffix, filename)
+    parser = app.registry.create_source_parser(app, filetype)
+    if parser.__class__.__name__ == 'CommonMarkParser' and parser.settings_spec == ():
+        # a workaround for recommonmark
+        #   If recommonmark.AutoStrictify is enabled, the parser invokes reST parser
+        #   internally.  But recommonmark-0.4.0 does not provide settings_spec for reST
+        #   parser.  As a workaround, this copies settings_spec for RSTParser to the
+        #   CommonMarkParser.
+        parser.settings_spec = RSTParser.settings_spec
+
+    pub = Publisher(reader=reader,
+                    parser=parser,
+                    writer=SphinxDummyWriter(),
+                    source_class=SphinxFileInput,
+                    destination=NullOutput())
+    pub.process_programmatic_settings(None, env.settings, None)
+    pub.set_source(source_path=filename)
+    pub.publish()
+    return pub.document
+
+
 def create_publisher(app: "Sphinx", filetype: str) -> Publisher:
     reader = SphinxStandaloneReader()
     reader.setup(app)