diff --git a/nixos/doc/manual/from_md/release-notes/rl-2205.section.xml b/nixos/doc/manual/from_md/release-notes/rl-2205.section.xml
index a1e7a35ab05..67c4355f50b 100644
--- a/nixos/doc/manual/from_md/release-notes/rl-2205.section.xml
+++ b/nixos/doc/manual/from_md/release-notes/rl-2205.section.xml
@@ -517,6 +517,13 @@
 }
 </programlisting>
       </listitem>
+      <listitem>
+        <para>
+          <literal>services.prometheus.alertManagerTimeout</literal> has
+          been removed as it has been deprecated upstream and has no
+          effect.
+        </para>
+      </listitem>
       <listitem>
         <para>
           The DHCP server (<literal>services.dhcpd4</literal>,
diff --git a/nixos/doc/manual/release-notes/rl-2205.section.md b/nixos/doc/manual/release-notes/rl-2205.section.md
index 20dbcf998d9..2c60e9ee780 100644
--- a/nixos/doc/manual/release-notes/rl-2205.section.md
+++ b/nixos/doc/manual/release-notes/rl-2205.section.md
@@ -173,6 +173,8 @@ In addition to numerous new and upgraded packages, this release has the followin
   }
   ```
 
+- `services.prometheus.alertManagerTimeout` has been removed as it has been deprecated upstream and has no effect.
+
 - The DHCP server (`services.dhcpd4`, `services.dhcpd6`) has been hardened.
   The service is now using the systemd's `DynamicUser` mechanism to run as an unprivileged dynamically-allocated user with limited capabilities.
   The dhcpd state files are now always stored in `/var/lib/dhcpd{4,6}` and the `services.dhcpd4.stateDir` and `service.dhcpd6.stateDir` options have been removed.
diff --git a/nixos/modules/services/monitoring/prometheus/default.nix b/nixos/modules/services/monitoring/prometheus/default.nix
index f563861b61c..52525e8935b 100644
--- a/nixos/modules/services/monitoring/prometheus/default.nix
+++ b/nixos/modules/services/monitoring/prometheus/default.nix
@@ -74,7 +74,6 @@ let
     }"
     "--web.listen-address=${cfg.listenAddress}:${builtins.toString cfg.port}"
     "--alertmanager.notification-queue-capacity=${toString cfg.alertmanagerNotificationQueueCapacity}"
-    "--alertmanager.timeout=${toString cfg.alertmanagerTimeout}s"
   ] ++ optional (cfg.webExternalUrl != null) "--web.external-url=${cfg.webExternalUrl}"
     ++ optional (cfg.retentionTime != null) "--storage.tsdb.retention.time=${cfg.retentionTime}";
 
@@ -1563,6 +1562,8 @@ in
     (mkRenamedOptionModule [ "services" "prometheus2" ] [ "services" "prometheus" ])
     (mkRemovedOptionModule [ "services" "prometheus" "environmentFile" ]
       "It has been removed since it was causing issues (https://github.com/NixOS/nixpkgs/issues/126083) and Prometheus now has native support for secret files, i.e. `basic_auth.password_file` and `authorization.credentials_file`.")
+    (mkRemovedOptionModule [ "services" "prometheus" "alertmanagerTimeout" ]
+      "Deprecated upstream and no longer had any effect")
   ];
 
   options.services.prometheus = {
@@ -1719,14 +1720,6 @@ in
       '';
     };
 
-    alertmanagerTimeout = mkOption {
-      type = types.int;
-      default = 10;
-      description = ''
-        Alert manager HTTP API timeout (in seconds).
-      '';
-    };
-
     webExternalUrl = mkOption {
       type = types.nullOr types.str;
       default = null;