diff --git a/ansible/roles/prometheus/defaults/main.yml b/ansible/roles/prometheus/defaults/main.yml
index bd34484655..10745bd99c 100644
--- a/ansible/roles/prometheus/defaults/main.yml
+++ b/ansible/roles/prometheus/defaults/main.yml
@@ -321,3 +321,5 @@ prometheus_memcached_exporter_cmdline_extras: ""
 prometheus_mysqld_exporter_cmdline_extras: ""
 prometheus_node_exporter_cmdline_extras: ""
 prometheus_openstack_exporter_cmdline_extras: ""
+
+prometheus_alertmanager_external_url: "{{ internal_protocol }}://{{ kolla_internal_fqdn | put_address_in_context('url') }}:{{ prometheus_alertmanager_port }}"
diff --git a/ansible/roles/prometheus/templates/prometheus-alertmanager.json.j2 b/ansible/roles/prometheus/templates/prometheus-alertmanager.json.j2
index 7453b5e453..06ab4132d0 100644
--- a/ansible/roles/prometheus/templates/prometheus-alertmanager.json.j2
+++ b/ansible/roles/prometheus/templates/prometheus-alertmanager.json.j2
@@ -1,5 +1,5 @@
 {
-    "command": "/opt/prometheus_alertmanager/alertmanager --config.file=/etc/prometheus/alertmanager.yml --web.listen-address={{ api_interface_address | put_address_in_context('url') }}:{{ prometheus_alertmanager_port }} --web.external-url={{ internal_protocol }}://{{ kolla_internal_fqdn | put_address_in_context('url') }}:{{ prometheus_alertmanager_port }} --cluster.listen-address={% if groups["prometheus-alertmanager"] | length > 1 %}{{ api_interface_address | put_address_in_context('url') }}:{{ prometheus_alertmanager_cluster_port }} {% for host in groups["prometheus-alertmanager"] %} --cluster.peer={{ 'api' | kolla_address(host) | put_address_in_context('url') }}:{{ hostvars[host]['prometheus_alertmanager_cluster_port'] }}{% endfor %}{% endif %} --storage.path /var/lib/prometheus",
+    "command": "/opt/prometheus_alertmanager/alertmanager --config.file=/etc/prometheus/alertmanager.yml --web.listen-address={{ api_interface_address | put_address_in_context('url') }}:{{ prometheus_alertmanager_port }} --web.external-url={{ prometheus_alertmanager_external_url }} --cluster.listen-address={% if groups["prometheus-alertmanager"] | length > 1 %}{{ api_interface_address | put_address_in_context('url') }}:{{ prometheus_alertmanager_cluster_port }} {% for host in groups["prometheus-alertmanager"] %} --cluster.peer={{ 'api' | kolla_address(host) | put_address_in_context('url') }}:{{ hostvars[host]['prometheus_alertmanager_cluster_port'] }}{% endfor %}{% endif %} --storage.path /var/lib/prometheus",
     "config_files": [
         {
             "source": "{{ container_config_directory }}/prometheus-alertmanager.yml",
diff --git a/releasenotes/notes/fix-hardcoded-alertmanager-weburl-9413b9e82a8610ea.yaml b/releasenotes/notes/fix-hardcoded-alertmanager-weburl-9413b9e82a8610ea.yaml
new file mode 100644
index 0000000000..07babad9ce
--- /dev/null
+++ b/releasenotes/notes/fix-hardcoded-alertmanager-weburl-9413b9e82a8610ea.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes an issue with AlertManager external Web URL being unconfigurable.
+    A new variable ``prometheus_alertmanager_external_url`` has been
+    introduced that users can use to set web.external-url to public.