diff --git a/qutebrowser/config/configtypes.py b/qutebrowser/config/configtypes.py
index 26e7cc789..fa02dc971 100644
--- a/qutebrowser/config/configtypes.py
+++ b/qutebrowser/config/configtypes.py
@@ -20,6 +20,7 @@
 """Setting options used for qutebrowser."""
 
 import re
+import json
 import shlex
 import base64
 import codecs
@@ -1423,6 +1424,61 @@ class UrlList(List):
                                                 "{}".format(val.errorString()))
 
 
+class HeaderDict(BaseType):
+
+    """A JSON-like dictionary for custom HTTP headers."""
+
+    def validate(self, value):
+        self._basic_validation(value)
+        if not value:
+            return
+
+        try:
+            json_val = json.loads(value)
+        except ValueError as e:
+            raise configexc.ValidationError(value, str(e))
+
+        if not isinstance(json_val, dict):
+            raise configexc.ValidationError(value, "Expected json dict, but "
+                                            "got {}".format(type(json_val)))
+
+        if not json_val:
+            if self.none_ok:
+                return
+            else:
+                raise configexc.ValidationError(value, "may not be empty!")
+
+        for key, val in json_val.items():
+            if not isinstance(key, str):
+                msg = "Expected string for key {!r} but got {}".format(
+                    key, type(key))
+                raise configexc.ValidationError(value, msg)
+
+            if not isinstance(val, str):
+                msg = "Expected string for value {!r} but got {}".format(
+                    key, type(key))
+                raise configexc.ValidationError(value, msg)
+
+            try:
+                key.encode('ascii')
+            except UnicodeEncodeError as e:
+                msg = "Key {!r} contains non-ascii characters: {}".format(
+                    key, e)
+                raise configexc.ValidationError(value, msg)
+
+            try:
+                val.encode('ascii')
+            except UnicodeEncodeError as e:
+                msg = "Value {!r} contains non-ascii characters: {}".format(
+                    val, e)
+                raise configexc.ValidationError(value, msg)
+
+
+    def transform(self, value):
+        val = json.loads(value)
+        return val or None
+
+
 class SessionName(BaseType):
 
     """The name of a session."""
diff --git a/tests/unit/config/test_configtypes.py b/tests/unit/config/test_configtypes.py
index a47c2f691..2c3f4a948 100644
--- a/tests/unit/config/test_configtypes.py
+++ b/tests/unit/config/test_configtypes.py
@@ -1715,6 +1715,41 @@ class TestSearchEngineName:
         assert klass().transform(val) == expected
 
 
+class TestHeaderDict:
+
+    @pytest.fixture
+    def klass(self):
+        return configtypes.HeaderDict
+
+    @pytest.mark.parametrize('val', [
+        '{"foo": "bar"}',
+        '{"foo": "bar", "baz": "fish"}',
+        '',  # empty value with none_ok=true
+        '{}',  # ditto
+    ])
+    def test_validate_valid(self, klass, val):
+        klass(none_ok=True).validate(val)
+
+    @pytest.mark.parametrize('val', [
+        '["foo"]',  # valid json but not a dict
+        '{"hello": 23}',  # non-string as value
+        '{"hällo": "world"}',  # non-ascii data in key
+        '{"hello": "wörld"}',  # non-ascii data in value
+        '',  # empty value with none_ok=False
+        '{}',  # ditto
+    ])
+    def test_validate_invalid(self, klass, val):
+        with pytest.raises(configexc.ValidationError):
+            klass().validate(val)
+
+    @pytest.mark.parametrize('val, expected', [
+        ('{"foo": "bar"}', {"foo": "bar"}),
+        ('{}', None),
+    ])
+    def test_transform(self, klass, val, expected):
+        assert klass(none_ok=True).transform(val) == expected
+
+
 class TestSearchEngineUrl:
 
     """Test SearchEngineUrl."""