diff --git a/cylc/flow/parsec/OrderedDict.py b/cylc/flow/parsec/OrderedDict.py
index ed5b85c20f4..8e727719b40 100644
--- a/cylc/flow/parsec/OrderedDict.py
+++ b/cylc/flow/parsec/OrderedDict.py
@@ -100,6 +100,19 @@ def prepend(self, key, value):
         self[key] = value
         self.move_to_end(key, last=False)
 
+    @staticmethod
+    def repl_val(target, replace, replacement):
+        """Replace dictionary values with a string.
+
+        Designed to be used recursively.
+        """
+        for key, val in target.items():
+            if isinstance(val, dict):
+                OrderedDictWithDefaults.repl_val(
+                    val, replace, replacement)
+            elif val == replace:
+                target[key] = replacement
+
 
 class DictTree:
     """An object providing a single point of access to a tree of dicts.
diff --git a/cylc/flow/parsec/config.py b/cylc/flow/parsec/config.py
index 29944c03b30..794ee9eea03 100644
--- a/cylc/flow/parsec/config.py
+++ b/cylc/flow/parsec/config.py
@@ -15,9 +15,11 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 from copy import deepcopy
+import json
 import re
+import sys
 from textwrap import dedent
-from typing import TYPE_CHECKING, Callable, Iterable, List, Optional
+from typing import TYPE_CHECKING, Callable, Iterable, List, Optional, TextIO
 
 from cylc.flow.context_node import ContextNode
 from cylc.flow.parsec.exceptions import (
@@ -33,10 +35,12 @@
 
 if TYPE_CHECKING:
     from optparse import Values
+    from typing_extensions import Literal
 
 
 class ParsecConfig:
     """Object wrapper for parsec functions."""
+    META: "Literal['meta']" = 'meta'
 
     def __init__(
         self,
@@ -162,7 +166,7 @@ def get(self, keys: Optional[Iterable[str]] = None, sparse: bool = False):
         return cfg
 
     def idump(self, items=None, sparse=False, prefix='',
-              oneline=False, none_str='', handle=None):
+              oneline=False, none_str='', handle=None, json=False):
         """
         items is a list of --item style inputs:
            '[runtime][foo]script'.
@@ -178,7 +182,40 @@ def idump(self, items=None, sparse=False, prefix='',
                 mkeys.append(j)
         if null:
             mkeys = [[]]
-        self.mdump(mkeys, sparse, prefix, oneline, none_str, handle=handle)
+        if json:
+            self.jdump(mkeys, sparse, oneline, none_str, handle=handle)
+        else:
+            self.mdump(mkeys, sparse, prefix, oneline, none_str, handle=handle)
+
+    def jdump(
+        self,
+        mkeys: Optional[Iterable] = None,
+        sparse: bool = False,
+        oneline: bool = False,
+        none_str: Optional[str] = None,
+        handle: Optional[TextIO] = None
+    ) -> None:
+        """Dump a config to JSON format.
+
+        Args:
+            mkeys: Items to display.
+            sparse: Only display user set items, not defaults.
+            oneline: Output on a single line.
+            none_str: Value to give instead of null.
+            handle: Where to write the output.
+        """
+        # Use json indent to control online output:
+        indent = None if oneline else 4
+
+        for keys in mkeys or []:
+            if not keys:
+                keys = []
+            cfg = self.get(keys, sparse)
+            if none_str:
+                OrderedDictWithDefaults.repl_val(cfg, None, none_str)
+            data = json.dumps(cfg, indent=indent)
+
+        print(data, file=handle or sys.stdout)
 
     def mdump(self, mkeys=None, sparse=False, prefix='',
               oneline=False, none_str='', handle=None):
diff --git a/cylc/flow/scripts/config.py b/cylc/flow/scripts/config.py
index aa7144a934d..b5826f46d07 100755
--- a/cylc/flow/scripts/config.py
+++ b/cylc/flow/scripts/config.py
@@ -110,6 +110,15 @@ def get_option_parser() -> COP:
             "overrides any settings it shares with those higher up."),
         action="store_true", default=False, dest="print_hierarchy")
 
+    parser.add_option(
+        '--json',
+        help=(
+            'Returns config as JSON rather than Cylc Config format.'),
+        default=False,
+        action='store_true',
+        dest='json'
+    )
+
     parser.add_option(icp_option)
 
     platform_listing_options_group = parser.add_option_group(
@@ -139,6 +148,28 @@ def get_option_parser() -> COP:
     return parser
 
 
+def json_opt_check(parser, options):
+    """Return an error if --json and incompatible options used.
+    """
+    not_with_json = {
+        '--print-hierarchy': 'print_hierarchy',
+        '--platform-names': 'print_platform_names',
+        '--platforms': 'print_platforms'
+    }
+
+    if not options.json:
+        return
+
+    not_with_json = [
+        name for name, dest
+        in not_with_json.items()
+        if options.__dict__[dest]]
+
+    if not_with_json:
+        parser.error(
+            f'--json incompatible with {" or ".join(not_with_json)}')
+
+
 def get_config_file_hierarchy(workflow_id: Optional[str] = None) -> List[str]:
     filepaths = [os.path.join(path, glbl_cfg().CONF_BASENAME)
                  for _, path in glbl_cfg().conf_dir_hierarchy]
@@ -163,6 +194,7 @@ async def _main(
     options: 'Values',
     *ids,
 ) -> None:
+    json_opt_check(parser, options)
 
     if options.print_platform_names and options.print_platforms:
         options.print_platform_names = False
@@ -188,7 +220,8 @@ async def _main(
             options.item,
             not options.defaults,
             oneline=options.oneline,
-            none_str=options.none_str
+            none_str=options.none_str,
+            json=options.json,
         )
         return
 
@@ -213,5 +246,6 @@ async def _main(
         options.item,
         not options.defaults,
         oneline=options.oneline,
-        none_str=options.none_str
+        none_str=options.none_str,
+        json=options.json
     )
diff --git a/tests/functional/cylc-config/11-json-dump.t b/tests/functional/cylc-config/11-json-dump.t
new file mode 100644
index 00000000000..5e2af26aa33
--- /dev/null
+++ b/tests/functional/cylc-config/11-json-dump.t
@@ -0,0 +1,113 @@
+#!/usr/bin/env bash
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#-------------------------------------------------------------------------------
+# Test cylc config can dump json files.
+# n.b. not heavily tested because most of this functionality
+# is from Standard library json.
+. "$(dirname "$0")/test_header"
+#-------------------------------------------------------------------------------
+set_test_number 9
+#-------------------------------------------------------------------------------
+
+# Test that option parser errors if incompat options given:
+cylc config --json --platforms 2> err
+named_grep_ok "${TEST_NAME_BASE}.CLI-one-incompat-item" \
+    "--json incompatible with --platforms" \
+    err
+
+cylc config --json --platforms --platform-names 2> err
+named_grep_ok "${TEST_NAME_BASE}.CLI-two-incompat-items" \
+    "--json incompatible with --platform-names or --platforms" \
+    err
+
+cylc config --json --platforms --platform-names --print-hierarchy 2> err
+named_grep_ok "${TEST_NAME_BASE}.CLI-three-incompat-items" \
+    "--json incompatible with --print-hierarchy or --platform-names or --platforms" \
+    err
+
+
+# Test the global.cylc
+TEST_NAME="${TEST_NAME_BASE}-global"
+
+cat > "global.cylc" <<__HEREDOC__
+[platforms]
+    [[golders_green]]
+        [[[meta]]]
+            can = "Test lots of things"
+            because = metadata, is, not, fussy
+            number = 99
+__HEREDOC__
+
+export CYLC_CONF_PATH="${PWD}"
+run_ok "${TEST_NAME}" cylc config --json --one-line
+cmp_ok "${TEST_NAME}.stdout" <<__HERE__
+{"platforms": {"golders_green": {"meta": {"can": "Test lots of things", "because": "metadata, is, not, fussy", "number": "99"}}}}
+__HERE__
+
+# Test a flow.cylc
+TEST_NAME="${TEST_NAME_BASE}-workflow"
+
+cat > "flow.cylc" <<__HERE__
+[scheduling]
+    [[graph]]
+        P1D = foo
+
+[runtime]
+    [[foo]]
+__HERE__
+
+run_ok "${TEST_NAME}" cylc config . --json --icp 1000
+cmp_ok "${TEST_NAME}.stdout" <<__HERE__
+{
+    "scheduling": {
+        "graph": {
+            "P1D": "foo"
+        },
+        "initial cycle point": "1000"
+    },
+    "runtime": {
+        "root": {},
+        "foo": {
+            "completion": "succeeded"
+        }
+    }
+}
+__HERE__
+
+# Test an empty global.cylc to check:
+# * item selection
+# * null value setting
+# * showing defaults
+TEST_NAME="${TEST_NAME_BASE}-defaults-item-null-value"
+echo "" > global.cylc
+export CYLC_CONF_PATH="${PWD}"
+
+run_ok "${TEST_NAME}" cylc config \
+    -i '[scheduler][mail]' \
+    --json \
+    --defaults \
+    --null-value='zilch'
+
+cmp_ok "${TEST_NAME}.stdout" <<__HERE__
+{
+    "from": "zilch",
+    "smtp": "zilch",
+    "to": "zilch",
+    "footer": "zilch",
+    "task event batch interval": 300.0
+}
+__HERE__