diff --git a/news/5359.bugfix b/news/5359.bugfix
new file mode 100644
index 00000000000..48b5fb7fb76
--- /dev/null
+++ b/news/5359.bugfix
@@ -0,0 +1,48 @@
+When executing a pip install from within a Python process in which the euid has
+been changed, the appdirs helpers which use `expanduser()` return paths
+relative to the home directory of the actual user, rather than the effective
+user. This causes the install to fail when the effective user cannot write to
+the cachedir:
+
+.. code-block:: python
+
+    >>> import subprocess
+    >>> import os
+    >>> os.geteuid()
+    0
+    >>> os.seteuid(1000)
+    >>> os.geteuid()
+    1000
+    >>> p = subprocess.Popen(['/home/erik/virtualenv/pipdev/bin/pip', 'install', 'pudb'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    >>> stdout, stderr = p.communicate()
+    >>> p.returncode
+    1
+    >>> stderr
+    "Could not install packages due to an EnvironmentError: [Errno 13] Permission denied: '/root/.cache/pip/wheels/ec/bb/79/09cd8a9982b637b251208ac3400f9702c72e63accc8e4b69e3'\nConsider using the `--user` option or check the permissions.\n\n"
+    >>>
+
+With this bugfix, the installation proceeds as expected, and the sources/wheels
+are downloaded to the effective user's cachedir:
+
+.. code-block:: python
+
+    >>> import subprocess
+    >>> import os
+    >>> os.geteuid()
+    0
+    >>> os.seteuid(1000)
+    >>> os.geteuid()
+    1000
+    >>> p = subprocess.Popen(['/home/erik/virtualenv/pipdev/bin/pip', 'install', 'pudb'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    >>> stdout, stderr = p.communicate()
+    >>> p.returncode
+    0
+    >>> print(stdout)
+    Collecting pudb
+    Collecting urwid>=1.1.1 (from pudb)
+    Collecting pygments>=1.0 (from pudb)
+      Using cached https://files.pythonhosted.org/packages/02/ee/b6e02dc6529e82b75bb06823ff7d005b141037cb1416b10c6f00fc419dca/Pygments-2.2.0-py2.py3-none-any.whl
+    Installing collected packages: urwid, pygments, pudb
+    Successfully installed pudb-2017.1.4 pygments-2.2.0 urwid-2.0.1
+
+    >>>
diff --git a/src/pip/_internal/utils/appdirs.py b/src/pip/_internal/utils/appdirs.py
index 28c5d4b3c75..d650524f31d 100644
--- a/src/pip/_internal/utils/appdirs.py
+++ b/src/pip/_internal/utils/appdirs.py
@@ -11,6 +11,18 @@
 
 from pip._internal.compat import WINDOWS, expanduser
 
+try:
+    import pwd
+except ImportError:
+    pwd = None
+
+
+def _effective_user():
+    try:
+        return pwd.getpwuid(os.geteuid()).pw_name
+    except Exception:
+        return ''
+
 
 def user_cache_dir(appname):
     r"""
@@ -44,15 +56,17 @@ def user_cache_dir(appname):
 
         # Add our app name and Cache directory to it
         path = os.path.join(path, appname, "Cache")
-    elif sys.platform == "darwin":
-        # Get the base path
-        path = expanduser("~/Library/Caches")
-
-        # Add our app name to it
-        path = os.path.join(path, appname)
     else:
-        # Get the base path
-        path = os.getenv("XDG_CACHE_HOME", expanduser("~/.cache"))
+        user = _effective_user()
+        if sys.platform == "darwin":
+            # Get the base path
+            path = expanduser("~{0}/Library/Caches".format(user))
+        else:
+            # Get the base path
+            path = os.getenv(
+                "XDG_CACHE_HOME",
+                expanduser("~{0}/.cache".format(user))
+            )
 
         # Add our app name to it
         path = os.path.join(path, appname)
@@ -91,23 +105,28 @@ def user_data_dir(appname, roaming=False):
     if WINDOWS:
         const = roaming and "CSIDL_APPDATA" or "CSIDL_LOCAL_APPDATA"
         path = os.path.join(os.path.normpath(_get_win_folder(const)), appname)
-    elif sys.platform == "darwin":
-        path = os.path.join(
-            expanduser('~/Library/Application Support/'),
-            appname,
-        ) if os.path.isdir(os.path.join(
-            expanduser('~/Library/Application Support/'),
-            appname,
-        )
-        ) else os.path.join(
-            expanduser('~/.config/'),
-            appname,
-        )
     else:
-        path = os.path.join(
-            os.getenv('XDG_DATA_HOME', expanduser("~/.local/share")),
-            appname,
-        )
+        user = _effective_user()
+        if sys.platform == "darwin":
+            path = os.path.join(
+                expanduser('~{0}/Library/Application Support/'.format(user)),
+                appname,
+            ) if os.path.isdir(os.path.join(
+                expanduser('~{0}/Library/Application Support/'.format(user)),
+                appname,
+            )
+            ) else os.path.join(
+                expanduser('~{0}/.config/'.format(user)),
+                appname,
+            )
+        else:
+            path = os.path.join(
+                os.getenv(
+                    'XDG_DATA_HOME',
+                    expanduser("~{0}/.local/share".format(user))
+                ),
+                appname,
+            )
 
     return path
 
@@ -137,7 +156,11 @@ def user_config_dir(appname, roaming=True):
     elif sys.platform == "darwin":
         path = user_data_dir(appname)
     else:
-        path = os.getenv('XDG_CONFIG_HOME', expanduser("~/.config"))
+        user = _effective_user()
+        path = os.getenv(
+            'XDG_CONFIG_HOME',
+            expanduser("~{0}/.config".format(user))
+        )
         path = os.path.join(path, appname)
 
     return path