diff --git a/changelogs/fragments/3933-slack-charset-header.yaml b/changelogs/fragments/3933-slack-charset-header.yaml
new file mode 100644
index 0000000000..581fd7a2e8
--- /dev/null
+++ b/changelogs/fragments/3933-slack-charset-header.yaml
@@ -0,0 +1,2 @@
+bugfixes:
+  - slack - add ``charset`` to HTTP headers to avoid Slack API warning (https://github.com/ansible-collections/community.general/issues/3932).
diff --git a/plugins/modules/notification/slack.py b/plugins/modules/notification/slack.py
index 3023bd9d8a..bdc839f9a8 100644
--- a/plugins/modules/notification/slack.py
+++ b/plugins/modules/notification/slack.py
@@ -346,7 +346,7 @@ def build_payload_for_slack(text, channel, thread_id, username, icon_url, icon_e
 
 def get_slack_message(module, token, channel, ts):
     headers = {
-        'Content-Type': 'application/json',
+        'Content-Type': 'application/json; charset=UTF-8',
         'Accept': 'application/json',
         'Authorization': 'Bearer ' + token
     }
@@ -383,7 +383,7 @@ def do_notify_slack(module, domain, token, payload):
         slack_uri = OLD_SLACK_INCOMING_WEBHOOK % (domain, token)
 
     headers = {
-        'Content-Type': 'application/json',
+        'Content-Type': 'application/json; charset=UTF-8',
         'Accept': 'application/json',
     }
     if use_webapi: