diff --git a/docs/config-file/v2.rst b/docs/config-file/v2.rst
index c559fafc90f..2de0411ae94 100644
--- a/docs/config-file/v2.rst
+++ b/docs/config-file/v2.rst
@@ -69,7 +69,7 @@ This is to avoid typos and provide feedback on invalid configurations.
 
 .. contents::
    :local:
-   :depth: 1
+   :depth: 3
 
 version
 ~~~~~~~
@@ -303,6 +303,9 @@ Configuration for the documentation build process.
 
    build:
      image: latest
+     apt_packages:
+       - libclang
+       - cmake
 
    python:
      version: 3.7
@@ -323,6 +326,32 @@ as defined here:
 * `stable <https://github.com/readthedocs/readthedocs-docker-images/tree/releases/5.x>`_: :buildpyversions:`stable`
 * `latest <https://github.com/readthedocs/readthedocs-docker-images/tree/releases/6.x>`_: :buildpyversions:`latest`
 
+build.apt_packages
+``````````````````
+
+List of `APT packages`_ to install.
+Our build servers run Ubuntu 18.04, with the default set of package repositories installed.
+We don't currently support PPA's or other custom repositories.
+
+.. _APT packages: https://packages.ubuntu.com/
+
+:Type: ``list``
+:Default: ``[]``
+
+.. code-block:: yaml
+
+   version: 2
+
+   build:
+     apt_packages:
+       - libclang
+       - cmake
+
+.. note::
+
+   When possible avoid installing Python packages using apt (``python3-numpy`` for example),
+   :ref:`use pip or Conda instead <guides/reproducible-builds:pinning dependencies>`.
+
 sphinx
 ~~~~~~
 
diff --git a/docs/guides/reproducible-builds.rst b/docs/guides/reproducible-builds.rst
index 485ec1ec2a1..0e0ed854b2f 100644
--- a/docs/guides/reproducible-builds.rst
+++ b/docs/guides/reproducible-builds.rst
@@ -173,6 +173,6 @@ or our Conda docs about :ref:`environment files <guides/conda:creating the \`\`e
 .. tip::
 
    Remember to update your docs' dependencies from time to time to get new improvements and fixes.
-   It also makes it easy to manage in case a version reaches it's end of support date.
+   It also makes it easy to manage in case a version reaches its end of support date.
 
    .. TODO: link to the supported versions policy.
diff --git a/readthedocs/config/config.py b/readthedocs/config/config.py
index 9166d171fc8..c1a3c4c8029 100644
--- a/readthedocs/config/config.py
+++ b/readthedocs/config/config.py
@@ -64,6 +64,7 @@
 SUBMODULES_INVALID = 'submodules-invalid'
 INVALID_KEYS_COMBINATION = 'invalid-keys-combination'
 INVALID_KEY = 'invalid-key'
+INVALID_NAME = 'invalid-name'
 
 LATEST_CONFIGURATION_VERSION = 2
 
@@ -121,10 +122,18 @@ def __init__(self, key, code, error_message, source_file=None):
         super().__init__(message, code=code)
 
     def _get_display_key(self):
-        # Checks for patterns similar to `python.install.0.requirements`
-        # if matched change to `python.install[0].requirements` using backreference.
+        """
+        Display keys in a more friendly format.
+
+        Indexes are displayed like ``n``,
+        but users may be more familiar with the ``[n]`` syntax.
+        For example ``python.install.0.requirements``
+        is changed to `python.install[0].requirements`.
+        """
         return re.sub(
-            r'^(python\.install)(\.)(\d+)(\.\w+)$', r'\1[\3]\4', self.key
+            r'^([a-zA-Z_.-]+)\.(\d+)([a-zA-Z_.-]*)$',
+            r'\1[\2]\3',
+            self.key
         )
 
 
@@ -745,12 +754,69 @@ def validate_build(self):
                 ),
             )
 
-        # Allow to override specific project
-        config_image = self.defaults.get('build_image')
-        if config_image:
-            build['image'] = config_image
+            # Allow to override specific project
+            config_image = self.defaults.get('build_image')
+            if config_image:
+                build['image'] = config_image
+
+        with self.catch_validation_error('build.apt_packages'):
+            raw_packages = self._raw_config.get('build', {}).get('apt_packages', [])
+            validate_list(raw_packages)
+            # Transform to a dict, so is easy to validate individual entries.
+            self._raw_config.setdefault('build', {})['apt_packages'] = (
+                list_to_dict(raw_packages)
+            )
+
+            build['apt_packages'] = [
+                self.validate_apt_package(index)
+                for index in range(len(raw_packages))
+            ]
+            if not raw_packages:
+                self.pop_config('build.apt_packages')
+
         return build
 
+    def validate_apt_package(self, index):
+        """
+        Validate the package name to avoid injections of extra options.
+
+        We validate that they aren't interpreted as an option or file.
+        See https://manpages.ubuntu.com/manpages/xenial/man8/apt-get.8.html
+        and https://www.debian.org/doc/manuals/debian-reference/ch02.en.html#_debian_package_file_names  # noqa
+        for allowed chars in packages names.
+        """
+        key = f'build.apt_packages.{index}'
+        package = self.pop_config(key)
+        with self.catch_validation_error(key):
+            validate_string(package)
+            package = package.strip()
+            invalid_starts = [
+                # Don't allow extra options.
+                '-',
+                # Don't allow to install from a path.
+                '/',
+                '.',
+            ]
+            for start in invalid_starts:
+                if package.startswith(start):
+                    self.error(
+                        key=key,
+                        message=(
+                            'Invalid package name. '
+                            f'Package can\'t start with {start}.',
+                        ),
+                        code=INVALID_NAME,
+                    )
+            # List of valid chars in packages names.
+            pattern = re.compile(r'^[a-zA-Z0-9][a-zA-Z0-9.+-]*$')
+            if not pattern.match(package):
+                self.error(
+                    key=key,
+                    message='Invalid package name.',
+                    code=INVALID_NAME,
+                )
+        return package
+
     def validate_python(self):
         """
         Validates the python key.
diff --git a/readthedocs/config/models.py b/readthedocs/config/models.py
index 998da8498be..714f9b80328 100644
--- a/readthedocs/config/models.py
+++ b/readthedocs/config/models.py
@@ -28,7 +28,11 @@ def as_dict(self):
 
 class Build(Base):
 
-    __slots__ = ('image',)
+    __slots__ = ('image', 'apt_packages')
+
+    def __init__(self, **kwargs):
+        kwargs.setdefault('apt_packages', [])
+        super().__init__(**kwargs)
 
 
 class Python(Base):
diff --git a/readthedocs/config/tests/test_config.py b/readthedocs/config/tests/test_config.py
index a2834c78305..c4bbe6dc069 100644
--- a/readthedocs/config/tests/test_config.py
+++ b/readthedocs/config/tests/test_config.py
@@ -26,6 +26,7 @@
     CONFIG_REQUIRED,
     CONFIG_SYNTAX_INVALID,
     INVALID_KEY,
+    INVALID_NAME,
     PYTHON_INVALID,
     VERSION_INVALID,
 )
@@ -748,6 +749,7 @@ def test_as_dict(tmpdir):
         },
         'build': {
             'image': 'readthedocs/build:latest',
+            'apt_packages': [],
         },
         'conda': None,
         'sphinx': {
@@ -935,6 +937,62 @@ def test_build_image_check_invalid_type(self, value):
             build.validate()
         assert excinfo.value.key == 'build.image'
 
+    @pytest.mark.parametrize(
+        'value',
+        [
+            [],
+            ['cmatrix'],
+            ['Mysql', 'cmatrix', 'postgresql-dev'],
+        ],
+    )
+    def test_build_apt_packages_check_valid(self, value):
+        build = self.get_build_config({'build': {'apt_packages': value}})
+        build.validate()
+        assert build.build.apt_packages == value
+
+    @pytest.mark.parametrize(
+        'value',
+        [3, 'string', {}],
+    )
+    def test_build_apt_packages_invalid_type(self, value):
+        build = self.get_build_config({'build': {'apt_packages': value}})
+        with raises(InvalidConfig) as excinfo:
+            build.validate()
+        assert excinfo.value.key == 'build.apt_packages'
+
+    @pytest.mark.parametrize(
+        'error_index, value',
+        [
+            (0, ['/', 'cmatrix']),
+            (1, ['cmatrix', '-q']),
+            (1, ['cmatrix', ' -q']),
+            (1, ['cmatrix', '\\-q']),
+            (1, ['cmatrix', '--quiet']),
+            (1, ['cmatrix', ' --quiet']),
+            (2, ['cmatrix', 'quiet', './package.deb']),
+            (2, ['cmatrix', 'quiet', ' ./package.deb ']),
+            (2, ['cmatrix', 'quiet', '/home/user/package.deb']),
+            (2, ['cmatrix', 'quiet', ' /home/user/package.deb']),
+            (2, ['cmatrix', 'quiet', '../package.deb']),
+            (2, ['cmatrix', 'quiet', ' ../package.deb']),
+            (1, ['one', '$two']),
+            (1, ['one', 'non-ascíí']),
+            # We don't allow regex for now.
+            (1, ['mysql', 'cmatrix$']),
+            (0, ['^mysql-*', 'cmatrix$']),
+            # We don't allow specifying versions for now.
+            (0, ['postgresql=1.2.3']),
+            # We don't allow specifying distributions for now.
+            (0, ['cmatrix/bionic']),
+        ],
+    )
+    def test_build_apt_packages_invalid_value(self, error_index, value):
+        build = self.get_build_config({'build': {'apt_packages': value}})
+        with raises(InvalidConfig) as excinfo:
+            build.validate()
+        assert excinfo.value.key == f'build.apt_packages.{error_index}'
+        assert excinfo.value.code == INVALID_NAME
+
     @pytest.mark.parametrize('value', [3, [], 'invalid'])
     def test_python_check_invalid_types(self, value):
         build = self.get_build_config({'python': value})
@@ -2072,6 +2130,7 @@ def test_as_dict(self, tmpdir):
             },
             'build': {
                 'image': 'readthedocs/build:latest',
+                'apt_packages': [],
             },
             'conda': None,
             'sphinx': {
diff --git a/readthedocs/projects/tasks.py b/readthedocs/projects/tasks.py
index 923ccf03f07..459f2cab047 100644
--- a/readthedocs/projects/tasks.py
+++ b/readthedocs/projects/tasks.py
@@ -794,7 +794,7 @@ def run_build(self, record):
                     environment=self.build_env,
                 )
                 with self.project.repo_nonblockinglock(version=self.version):
-                    self.setup_python_environment()
+                    self.setup_build()
 
                     # TODO the build object should have an idea of these states,
                     # extend the model to include an idea of these outcomes
@@ -1152,6 +1152,10 @@ def update_app_instances(
             search_ignore=self.config.search.ignore,
         )
 
+    def setup_build(self):
+        self.install_system_dependencies()
+        self.setup_python_environment()
+
     def setup_python_environment(self):
         """
         Build the virtualenv and install the project into it.
@@ -1177,6 +1181,30 @@ def setup_python_environment(self):
         if self.project.has_feature(Feature.LIST_PACKAGES_INSTALLED_ENV):
             self.python_env.list_packages_installed()
 
+    def install_system_dependencies(self):
+        """
+        Install apt packages from the config file.
+
+        We don't allow to pass custom options or install from a path.
+        The packages names are already validated when reading the config file.
+
+        .. note::
+
+           ``--quiet`` won't suppress the output,
+           it would just remove the progress bar.
+        """
+        packages = self.config.build.apt_packages
+        if packages:
+            self.build_env.run(
+                'apt-get', 'update', '--assume-yes', '--quiet',
+                user=settings.RTD_DOCKER_SUPER_USER,
+            )
+            # put ``--`` to end all command arguments.
+            self.build_env.run(
+                'apt-get', 'install', '--assume-yes', '--quiet', '--', *packages,
+                user=settings.RTD_DOCKER_SUPER_USER,
+            )
+
     def build_docs(self):
         """
         Wrapper to all build functions.
diff --git a/readthedocs/rtd_tests/fixtures/spec/v2/schema.yml b/readthedocs/rtd_tests/fixtures/spec/v2/schema.yml
index f88db8025d1..70907d34e14 100644
--- a/readthedocs/rtd_tests/fixtures/spec/v2/schema.yml
+++ b/readthedocs/rtd_tests/fixtures/spec/v2/schema.yml
@@ -50,6 +50,10 @@ build:
   # Note: it can be overridden by a project
   image: enum('stable', 'latest', required=False)
 
+  # List of packages to be installed with apt-get
+  # Default: []
+  apt_packages: list(str(), required=False)
+
 python:
   # The Python version (this depends on the build image)
   # Default: '3'
diff --git a/readthedocs/rtd_tests/tests/test_celery.py b/readthedocs/rtd_tests/tests/test_celery.py
index 6f684f3b0d3..b675d81d923 100644
--- a/readthedocs/rtd_tests/tests/test_celery.py
+++ b/readthedocs/rtd_tests/tests/test_celery.py
@@ -2,6 +2,7 @@
 import shutil
 from os.path import exists
 from tempfile import mkdtemp
+from unittest import mock
 from unittest.mock import MagicMock, patch
 
 from allauth.socialaccount.models import SocialAccount
@@ -17,7 +18,11 @@
     LATEST,
 )
 from readthedocs.builds.models import Build, Version
-from readthedocs.doc_builder.environments import LocalBuildEnvironment
+from readthedocs.config.config import BuildConfigV2
+from readthedocs.doc_builder.environments import (
+    BuildEnvironment,
+    LocalBuildEnvironment,
+)
 from readthedocs.doc_builder.exceptions import VersionLockedError
 from readthedocs.oauth.models import RemoteRepository, RemoteRepositoryRelation
 from readthedocs.projects import tasks
@@ -469,3 +474,67 @@ def test_send_build_status_no_remote_repo_or_social_account_gitlab(self, send_bu
 
         send_build_status.assert_not_called()
         self.assertEqual(Message.objects.filter(user=self.eric).count(), 1)
+
+    @patch('readthedocs.projects.tasks.UpdateDocsTaskStep.setup_python_environment', new=MagicMock)
+    @patch('readthedocs.projects.tasks.UpdateDocsTaskStep.build_docs', new=MagicMock)
+    @patch('readthedocs.projects.tasks.UpdateDocsTaskStep.setup_vcs', new=MagicMock)
+    @patch.object(BuildEnvironment, 'run')
+    @patch('readthedocs.doc_builder.config.load_config')
+    def test_install_apt_packages(self, load_config, run):
+        config = BuildConfigV2(
+            {},
+            {
+                'version': 2,
+                'build': {
+                    'apt_packages': [
+                        'clangd',
+                        'cmatrix',
+                    ],
+                },
+            },
+            source_file='readthedocs.yml',
+        )
+        config.validate()
+        load_config.return_value = config
+
+        version = self.project.versions.first()
+        build = get(
+            Build,
+            project=self.project,
+            version=version,
+        )
+        with mock_api(self.repo):
+            result = tasks.update_docs_task.delay(
+                version.pk,
+                build_pk=build.pk,
+                record=False,
+                intersphinx=False,
+            )
+        self.assertTrue(result.successful())
+
+        self.assertEqual(run.call_count, 2)
+        apt_update = run.call_args_list[0]
+        apt_install = run.call_args_list[1]
+        self.assertEqual(
+            apt_update,
+            mock.call(
+                'apt-get',
+                'update',
+                '--assume-yes',
+                '--quiet',
+                user='root:root',
+            )
+        )
+        self.assertEqual(
+            apt_install,
+            mock.call(
+                'apt-get',
+                'install',
+                '--assume-yes',
+                '--quiet',
+                '--',
+                'clangd',
+                'cmatrix',
+                user='root:root',
+            )
+        )
diff --git a/readthedocs/settings/base.py b/readthedocs/settings/base.py
index ef3c002b4c0..66069e26bde 100644
--- a/readthedocs/settings/base.py
+++ b/readthedocs/settings/base.py
@@ -440,6 +440,7 @@ def TEMPLATES(self):
     # instance to avoid file permissions issues.
     # https://docs.docker.com/engine/reference/run/#user
     RTD_DOCKER_USER = 'docs:docs'
+    RTD_DOCKER_SUPER_USER = 'root:root'
 
     RTD_DOCKER_COMPOSE = False