diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -1872,6 +1872,7 @@
     for section, name, value in ui.walkconfig(untrusted=untrusted):
         source = ui.configsource(section, name, untrusted)
         value = pycompat.bytestr(value)
+        defaultvalue = pycompat.bytestr(ui.configdefault(section, name))
         if fm.isplain():
             source = source or 'none'
             value = value.replace('\n', '\\n')
@@ -1881,7 +1882,7 @@
         fm.startitem()
         fm.condwrite(ui.debugflag, 'source', '%s: ', source)
         if uniquesel:
-            fm.data(name=entryname)
+            fm.data(name=entryname, defaultvalue=defaultvalue)
             fm.write('value', '%s\n', value)
         else:
             fm.write('name value', '%s=%s\n', entryname, value)
diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -783,6 +783,17 @@
             return None
         return default
 
+    def configdefault(self, section, name):
+        """returns the default value of the config item"""
+        item = self._knownconfig.get(section, {}).get(name)
+        itemdefault = None
+        if item is not None:
+            if callable(item.default):
+                itemdefault = item.default()
+            else:
+                itemdefault = item.default
+        return itemdefault
+
     def hasconfig(self, section, name, untrusted=False):
         return self._data(untrusted).hasitem(section, name)
 
diff --git a/tests/test-config.t b/tests/test-config.t
--- a/tests/test-config.t
+++ b/tests/test-config.t
@@ -70,6 +70,7 @@
   $ hg showconfig Section.KeY -Tjson
   [
    {
+    "defaultvalue": "None",
     "name": "Section.KeY",
     "source": "*.hgrc:*", (glob)
     "value": "Case Sensitive"
@@ -102,6 +103,7 @@
   $ hg config empty.source -Tjson
   [
    {
+    "defaultvalue": "None",
     "name": "empty.source",
     "source": "",
     "value": "value"