diff --git a/changes.d/6509.feat.md b/changes.d/6509.feat.md
new file mode 100644
index 0000000000..8406379562
--- /dev/null
+++ b/changes.d/6509.feat.md
@@ -0,0 +1 @@
+Added --global flag to 'cylc reload'
diff --git a/cylc/flow/cfgspec/globalcfg.py b/cylc/flow/cfgspec/globalcfg.py
index 18570a65c3..06b7265ec7 100644
--- a/cylc/flow/cfgspec/globalcfg.py
+++ b/cylc/flow/cfgspec/globalcfg.py
@@ -108,6 +108,13 @@
 
    Not to be confused with :cylc:conf:`flow.cylc[scheduling]`.
 
+.. note::
+
+   The majority of scheduler settings affect the server and cannot be reloaded
+   with ``cylc reload --global``, the server must be stopped and restarted for
+   changes to take effect except for the sections [mail] and [events] which
+   provide workflow defaults.
+
 .. versionchanged:: 8.0.0
 
    {REPLACES}``[cylc]``
@@ -1171,6 +1178,11 @@ def default_for(
             Symlinks from the the standard ``$HOME/cylc-run`` locations will be
             created.
 
+            .. note::
+
+               Once a platform has been installed and symlinks created they
+               cannot be modified for that run.
+
             .. versionadded:: 8.0.0
         """):
             with Conf('<install target>', desc=dedent("""
diff --git a/cylc/flow/commands.py b/cylc/flow/commands.py
index c45194752a..823022f52d 100644
--- a/cylc/flow/commands.py
+++ b/cylc/flow/commands.py
@@ -85,6 +85,7 @@
 from cylc.flow.parsec.exceptions import ParsecError
 from cylc.flow.run_modes import RunMode
 from cylc.flow.task_id import TaskID
+from cylc.flow.cfgspec.glbl_cfg import glbl_cfg
 from cylc.flow.workflow_status import StopMode
 
 
@@ -327,7 +328,7 @@ async def remove_tasks(
 
 
 @_command('reload_workflow')
-async def reload_workflow(schd: 'Scheduler'):
+async def reload_workflow(schd: 'Scheduler', reload_global: bool = False):
     """Reload workflow configuration."""
     yield
     # pause the workflow if not already
@@ -361,6 +362,25 @@ async def reload_workflow(schd: 'Scheduler'):
         # give commands time to complete
         sleep(1)  # give any remove-init's time to complete
 
+    if reload_global:
+        # Reload global config if requested
+        LOG.info("Reloading the global configuration.")
+        try:
+            glbl_cfg(reload=True)
+        except (ParsecError, CylcConfigError) as exc:
+            if cylc.flow.flags.verbosity > 1:
+                # log full traceback in debug mode
+                LOG.exception(exc)
+            LOG.critical(
+                f'Reload failed - {exc.__class__.__name__}: {exc}'
+                '\nThis is probably due to an issue with the new'
+                ' configuration.'
+                '\nTo continue with the pre-reload config, un-pause the'
+                ' workflow.'
+                '\nOtherwise, fix the configuration and attempt to reload'
+                ' again.'
+            )
+
     # reload the workflow definition
     schd.reload_pending = 'loading the workflow definition'
     schd.update_data_store()  # update workflow status msg
diff --git a/cylc/flow/network/schema.py b/cylc/flow/network/schema.py
index 3f6534b418..5f2d6c7598 100644
--- a/cylc/flow/network/schema.py
+++ b/cylc/flow/network/schema.py
@@ -1919,6 +1919,10 @@ class Meta:
     class Arguments:
         workflows = graphene.List(WorkflowID, required=True)
 
+        reload_global = Boolean(
+            default_value=False,
+            description="Also reload global config")
+
     result = GenericScalar()
 
 
diff --git a/cylc/flow/scripts/reload.py b/cylc/flow/scripts/reload.py
index ca11062e9b..c8e39989d1 100755
--- a/cylc/flow/scripts/reload.py
+++ b/cylc/flow/scripts/reload.py
@@ -60,6 +60,7 @@
 from cylc.flow.option_parsers import (
     WORKFLOW_ID_MULTI_ARG_DOC,
     CylcOptionParser as COP,
+    OptionSettings,
 )
 from cylc.flow.terminal import cli_function
 
@@ -67,12 +68,26 @@
     from optparse import Values
 
 
+RELOAD_OPTIONS = [
+    OptionSettings(
+        ['-g', '--global'],
+        help='also reload global configuration.',
+        action="store_true",
+        default=False,
+        dest="reload_global",
+        sources={'reload'}
+    ),
+]
+
+
 MUTATION = '''
 mutation (
-  $wFlows: [WorkflowID]!
+  $wFlows: [WorkflowID]!,
+  $reloadGlobal: Boolean,
 ) {
   reload (
     workflows: $wFlows
+    reloadGlobal: $reloadGlobal
   ) {
     result
   }
@@ -87,6 +102,9 @@ def get_option_parser():
         multiworkflow=True,
         argdoc=[WORKFLOW_ID_MULTI_ARG_DOC],
     )
+    for option in RELOAD_OPTIONS:
+        parser.add_option(*option.args, **option.kwargs)
+
     return parser
 
 
@@ -97,6 +115,7 @@ async def run(options: 'Values', workflow_id: str):
         'request_string': MUTATION,
         'variables': {
             'wFlows': [workflow_id],
+            'reloadGlobal': options.reload_global,
         }
     }
 
diff --git a/cylc/flow/scripts/validate_reinstall.py b/cylc/flow/scripts/validate_reinstall.py
index 4e78d673d7..afb36ddeab 100644
--- a/cylc/flow/scripts/validate_reinstall.py
+++ b/cylc/flow/scripts/validate_reinstall.py
@@ -73,6 +73,7 @@
     reinstall_cli as cylc_reinstall,
 )
 from cylc.flow.scripts.reload import (
+    RELOAD_OPTIONS,
     run as cylc_reload
 )
 from cylc.flow.terminal import cli_function
@@ -86,6 +87,7 @@
     VALIDATE_OPTIONS,
     REINSTALL_OPTIONS,
     REINSTALL_CYLC_ROSE_OPTIONS,
+    RELOAD_OPTIONS,
     PLAY_OPTIONS,
     CYLC_ROSE_OPTIONS,
     modify={'cylc-rose': 'validate, install'}
@@ -102,6 +104,7 @@ def get_option_parser() -> COP:
     for option in VR_OPTIONS:
         parser.add_option(*option.args, **option.kwargs)
     parser.set_defaults(is_validate=True)
+
     return parser
 
 
diff --git a/tests/functional/reload/29-global.t b/tests/functional/reload/29-global.t
new file mode 100644
index 0000000000..2ffa716b0b
--- /dev/null
+++ b/tests/functional/reload/29-global.t
@@ -0,0 +1,47 @@
+#!/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 reloading global configuration
+. "$(dirname "$0")/test_header"
+set_test_number 9
+
+create_test_global_config "" ""
+
+TEST_NAME="${TEST_NAME_BASE}"
+install_workflow "${TEST_NAME}" "${TEST_NAME_BASE}"
+
+# Validate the config
+run_ok "${TEST_NAME}-validate" cylc validate "${WORKFLOW_NAME}"
+
+# Run the workflow
+workflow_run_ok "${TEST_NAME_BASE}-run" cylc play "${WORKFLOW_NAME}" --no-detach -v
+
+# Reload happened
+grep_ok "Reloading the global configuration" "${WORKFLOW_RUN_DIR}/log/scheduler/01-start-01.log"
+
+# Reload hasn't happened in job a, has happened in b and c
+grep_fail "global init-script reloaded!" "${WORKFLOW_RUN_DIR}/log/job/1/a/01/job.out"
+grep_ok "global init-script reloaded!" "${WORKFLOW_RUN_DIR}/log/job/1/b/01/job.out"
+grep_ok "global init-script reloaded!" "${WORKFLOW_RUN_DIR}/log/job/1/c/01/job.out"
+
+# Events are original in job a, updated in b and c
+grep_fail "!!EVENT!! succeeded 1/a" "${WORKFLOW_RUN_DIR}/log/scheduler/01-start-01.log"
+grep_ok "!!EVENT!! succeeded 1/b" "${WORKFLOW_RUN_DIR}/log/scheduler/01-start-01.log"
+grep_ok "!!EVENT!! succeeded 1/c" "${WORKFLOW_RUN_DIR}/log/scheduler/01-start-01.log"
+
+purge
+exit
diff --git a/tests/functional/reload/29-global/flow.cylc b/tests/functional/reload/29-global/flow.cylc
new file mode 100644
index 0000000000..e1584c977a
--- /dev/null
+++ b/tests/functional/reload/29-global/flow.cylc
@@ -0,0 +1,30 @@
+[scheduling]
+    cycling mode = integer
+    [[graph]]
+        R1 = a => reload-global => b & c
+[runtime]
+    [[root]]
+        script = true
+    [[a,b]]
+        platform = localhost
+    [[c]]
+        platform = $CYLC_TEST_PLATFORM
+    [[reload-global]]
+        platform = localhost
+        script = """
+            # Append to global config
+            cat >> "$CYLC_CONF_PATH/global.cylc" <<EOF 
+                [platforms]
+                    [[localhost,${CYLC_TEST_PLATFORM}]]
+                        global init-script = echo "global init-script reloaded!"
+                [task events]
+                    handlers = echo "!!EVENT!! %(event)s %(id)s"
+                    handler events = succeeded
+EOF
+
+            # Log the new config
+            cylc config
+
+            # Reload the global config
+            cylc reload --global "${CYLC_WORKFLOW_ID}"
+            """
diff --git a/tests/integration/test_reload.py b/tests/integration/test_reload.py
index ad96b18772..3d17a9f35e 100644
--- a/tests/integration/test_reload.py
+++ b/tests/integration/test_reload.py
@@ -24,6 +24,9 @@
     TASK_STATUS_PREPARING,
     TASK_STATUS_SUBMITTED,
 )
+from cylc.flow.cfgspec.glbl_cfg import glbl_cfg
+from cylc.flow.platforms import get_platform
+from cylc.flow import flags
 
 
 async def test_reload_waits_for_pending_tasks(
@@ -146,3 +149,167 @@ async def test_reload_failure(
 
         # the config should be unchanged
         assert schd.config.cfg['scheduling']['graph']['R1'] == 'one'
+
+
+async def test_reload_global_platform(
+    flow,
+    one_conf,
+    scheduler,
+    start,
+    log_filter,
+    tmp_path,
+    monkeypatch,
+    ):
+
+    global_config_path = tmp_path / 'global.cylc'
+    monkeypatch.setenv("CYLC_CONF_PATH", str(global_config_path.parent))
+
+    # Original global config file
+    global_config_path.write_text("""
+    [platforms]
+        [[localhost]]
+            [[[meta]]]
+                x = 1
+    """)
+    glbl_cfg(reload=True)
+    assert glbl_cfg().get(['platforms','localhost','meta','x']) == '1'
+
+    id_ = flow(one_conf)
+    schd = scheduler(id_)
+    async with start(schd):
+        # Task platforms reflect the original config
+        rtconf = schd.broadcast_mgr.get_updated_rtconfig(schd.pool.get_tasks()[0])
+        platform = get_platform(rtconf)
+        assert platform['meta']['x'] == '1'
+
+        # Modify the global config file
+        global_config_path.write_text("""
+        [platforms]
+            [[localhost]]
+                [[[meta]]]
+                    x = 2
+        """)
+
+        # reload the workflow and global config
+        await commands.run_cmd(commands.reload_workflow(schd, reload_global=True))
+
+        # Global config should have been reloaded
+        assert log_filter(
+            contains=(
+                'Reloading the global configuration.'
+            )
+        )
+
+        # Task platforms reflect the new config
+        rtconf = schd.broadcast_mgr.get_updated_rtconfig(schd.pool.get_tasks()[0])
+        platform = get_platform(rtconf)
+        assert platform['meta']['x'] == '2'
+
+        # Modify the global config file with an error
+        global_config_path.write_text("""
+        [ERROR]
+        [platforms]
+            [[localhost]]
+                [[[meta]]]
+                    x = 3
+        """)
+
+        # reload the workflow and global config
+        await commands.run_cmd(commands.reload_workflow(schd, reload_global=True))
+
+        # Error is noted in the log
+        assert log_filter(
+            contains=(
+                'This is probably due to an issue with the new configuration.'
+            )
+        )
+
+        # Task platforms should be the last valid value
+        rtconf = schd.broadcast_mgr.get_updated_rtconfig(schd.pool.get_tasks()[0])
+        platform = get_platform(rtconf)
+        assert platform['meta']['x'] == '2'
+
+        # reload the workflow in verbose mode
+        flags.verbosity = 2
+        await commands.run_cmd(commands.reload_workflow(schd, reload_global=True))
+
+        # Traceback is shown in the log (match for ERROR, trace is not captured)
+        assert log_filter(
+            exact_match = 'ERROR'
+        )
+
+
+async def test_reload_global_platform_group(
+    flow,
+    scheduler,
+    start,
+    log_filter,
+    tmp_path,
+    monkeypatch,
+    ):
+
+    global_config_path = tmp_path / 'global.cylc'
+    monkeypatch.setenv("CYLC_CONF_PATH", str(global_config_path.parent))
+
+    # Original global config file
+    global_config_path.write_text("""
+    [platforms]
+        [[foo]]
+            [[[meta]]]
+                x = 1
+    [platform groups]
+        [[pg]]
+            platforms = foo
+    """)
+    glbl_cfg(reload=True)
+
+    # Task using the platform group
+    conf = {
+        'scheduler': {
+            'allow implicit tasks': True
+        },
+        'scheduling': {
+            'graph': {
+                'R1': 'one'
+            }
+        },
+        'runtime': {
+            'one': {
+                'platform': 'pg',
+            }
+        },
+    }
+
+    id_ = flow(conf)
+    schd = scheduler(id_)
+    async with start(schd):
+        # Task platforms reflect the original config
+        rtconf = schd.broadcast_mgr.get_updated_rtconfig(schd.pool.get_tasks()[0])
+        platform = get_platform(rtconf)
+        assert platform['meta']['x'] == '1'
+
+        # Modify the global config file
+        global_config_path.write_text("""
+        [platforms]
+            [[bar]]
+                [[[meta]]]
+                    x = 2
+        [platform groups]
+            [[pg]]
+                platforms = bar
+        """)
+
+        # reload the workflow and global config
+        await commands.run_cmd(commands.reload_workflow(schd, reload_global=True))
+
+        # Global config should have been reloaded
+        assert log_filter(
+            contains=(
+                'Reloading the global configuration.'
+            )
+        )
+
+        # Task platforms reflect the new config
+        rtconf = schd.broadcast_mgr.get_updated_rtconfig(schd.pool.get_tasks()[0])
+        platform = get_platform(rtconf)
+        assert platform['meta']['x'] == '2'