diff --git a/nixos/modules/services/web-apps/mattermost.nix b/nixos/modules/services/web-apps/mattermost.nix
index f5c2c356afce..a3127aed6925 100644
--- a/nixos/modules/services/web-apps/mattermost.nix
+++ b/nixos/modules/services/web-apps/mattermost.nix
@@ -6,23 +6,18 @@ let
 
   cfg = config.services.mattermost;
 
-  defaultConfig = builtins.fromJSON (builtins.replaceStrings [ "\\u0026" ] [ "&" ]
-    (readFile "${pkgs.mattermost}/config/config.json")
-  );
-
   database = "postgres://${cfg.localDatabaseUser}:${cfg.localDatabasePassword}@localhost:5432/${cfg.localDatabaseName}?sslmode=disable&connect_timeout=10";
 
-  mattermostConf = foldl recursiveUpdate defaultConfig
-    [ { ServiceSettings.SiteURL = cfg.siteUrl;
-        ServiceSettings.ListenAddress = cfg.listenAddress;
-        TeamSettings.SiteName = cfg.siteName;
-        SqlSettings.DriverName = "postgres";
-        SqlSettings.DataSource = database;
-      }
-      cfg.extraConfig
-    ];
+  mattermostConf = recursiveUpdate
+    { ServiceSettings.SiteURL = cfg.siteUrl;
+      ServiceSettings.ListenAddress = cfg.listenAddress;
+      TeamSettings.SiteName = cfg.siteName;
+      SqlSettings.DriverName = "postgres";
+      SqlSettings.DataSource = database;
+    }
+    cfg.extraConfig;
 
-  mattermostConfJSON = pkgs.writeText "mattermost-config-raw.json" (builtins.toJSON mattermostConf);
+  mattermostConfJSON = pkgs.writeText "mattermost-config.json" (builtins.toJSON mattermostConf);
 
 in
 
@@ -77,6 +72,16 @@ in
         '';
       };
 
+      preferNixConfig = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          If both mutableConfig and this option are set, the Nix configuration
+          will take precedence over any settings configured in the server
+          console.
+        '';
+      };
+
       extraConfig = mkOption {
         type = types.attrs;
         default = { };
@@ -180,14 +185,19 @@ in
           ln -sf ${pkgs.mattermost}/{bin,fonts,i18n,templates,client} ${cfg.statePath}
         '' + lib.optionalString (!cfg.mutableConfig) ''
           rm -f ${cfg.statePath}/config/config.json
-          cp ${mattermostConfJSON} ${cfg.statePath}/config/config.json
+          ${pkgs.jq}/bin/jq -s '.[0] * .[1]' ${pkgs.mattermost}/config/config.json ${mattermostConfJSON} > ${cfg.statePath}/config/config.json
           ${pkgs.mattermost}/bin/mattermost config migrate ${cfg.statePath}/config/config.json ${database}
         '' + lib.optionalString cfg.mutableConfig ''
           if ! test -e "${cfg.statePath}/config/.initial-created"; then
             rm -f ${cfg.statePath}/config/config.json
-            cp ${mattermostConfJSON} ${cfg.statePath}/config/config.json
+            ${pkgs.jq}/bin/jq -s '.[0] * .[1]' ${pkgs.mattermost}/config/config.json ${mattermostConfJSON} > ${cfg.statePath}/config/config.json
             touch ${cfg.statePath}/config/.initial-created
           fi
+        '' + lib.optionalString (cfg.mutableConfig && cfg.preferNixConfig) ''
+          newConfig="$(${pkgs.jq}/bin/jq -s '.[0] * .[1]' ${cfg.statePath}/config/config.json ${mattermostConfJSON})"
+
+          rm -f ${cfg.statePath}/config/config.json
+          echo "$newConfig" > ${cfg.statePath}/config/config.json
         '' + lib.optionalString cfg.localDatabaseCreate ''
           if ! test -e "${cfg.statePath}/.db-created"; then
             ${pkgs.sudo}/bin/sudo -u ${config.services.postgresql.superUser} \
diff --git a/nixos/tests/all-tests.nix b/nixos/tests/all-tests.nix
index db526df392b3..42f9c07703d5 100644
--- a/nixos/tests/all-tests.nix
+++ b/nixos/tests/all-tests.nix
@@ -261,6 +261,7 @@ in
   matomo = handleTest ./matomo.nix {};
   matrix-appservice-irc = handleTest ./matrix-appservice-irc.nix {};
   matrix-synapse = handleTest ./matrix-synapse.nix {};
+  mattermost = handleTest ./mattermost.nix {};
   mediawiki = handleTest ./mediawiki.nix {};
   meilisearch = handleTest ./meilisearch.nix {};
   memcached = handleTest ./memcached.nix {};
diff --git a/nixos/tests/mattermost.nix b/nixos/tests/mattermost.nix
new file mode 100644
index 000000000000..116d15e1315e
--- /dev/null
+++ b/nixos/tests/mattermost.nix
@@ -0,0 +1,116 @@
+import ./make-test-python.nix ({ pkgs, lib, ... }:
+let
+  host = "smoke.test";
+  port = "8065";
+  url = "http://${host}:${port}";
+  siteName = "NixOS Smoke Tests, Inc.";
+
+  makeMattermost = mattermostConfig:
+    { config, ... }: {
+      environment.systemPackages = [
+        pkgs.mattermost
+        pkgs.curl
+        pkgs.jq
+      ];
+      networking.hosts = {
+        "127.0.0.1" = [ host ];
+      };
+      services.mattermost = lib.recursiveUpdate {
+        enable = true;
+        inherit siteName;
+        listenAddress = "0.0.0.0:${port}";
+        siteUrl = url;
+        extraConfig = {
+          SupportSettings.AboutLink = "https://nixos.org";
+        };
+      } mattermostConfig;
+    };
+in
+{
+  name = "mattermost";
+
+  nodes = {
+    mutable = makeMattermost {
+      mutableConfig = true;
+      extraConfig.SupportSettings.HelpLink = "https://search.nixos.org";
+    };
+    mostlyMutable = makeMattermost {
+      mutableConfig = true;
+      preferNixConfig = true;
+    };
+    immutable = makeMattermost {
+      mutableConfig = false;
+      extraConfig.SupportSettings.HelpLink = "https://search.nixos.org";
+    };
+  };
+
+  testScript = let
+    expectConfig = jqExpression: pkgs.writeShellScript "expect-config" ''
+      set -euo pipefail
+      echo "Expecting config to match: "${lib.escapeShellArg jqExpression} >&2
+      curl ${lib.escapeShellArg url} >/dev/null
+      config="$(curl ${lib.escapeShellArg "${url}/api/v4/config/client?format=old"})"
+      echo "Config: $(echo "$config" | ${pkgs.jq}/bin/jq)" >&2
+      [[ "$(echo "$config" | ${pkgs.jq}/bin/jq -r ${lib.escapeShellArg ".SiteName == $siteName and .Version == ($mattermostName / $sep)[-1] and (${jqExpression})"} --arg siteName ${lib.escapeShellArg siteName} --arg mattermostName ${lib.escapeShellArg pkgs.mattermost.name} --arg sep '-')" = "true" ]]
+    '';
+
+    setConfig = jqExpression: pkgs.writeShellScript "set-config" ''
+      set -euo pipefail
+      mattermostConfig=/var/lib/mattermost/config/config.json
+      newConfig="$(${pkgs.jq}/bin/jq -r ${lib.escapeShellArg jqExpression} $mattermostConfig)"
+      rm -f $mattermostConfig
+      echo "$newConfig" > "$mattermostConfig"
+    '';
+  in
+  ''
+    start_all()
+
+    ## Mutable node tests ##
+    mutable.wait_for_unit("mattermost.service")
+    mutable.wait_for_open_port(8065)
+
+    # Get the initial config
+    mutable.succeed("${expectConfig ''.AboutLink == "https://nixos.org" and .HelpLink == "https://search.nixos.org"''}")
+
+    # Edit the config
+    mutable.succeed("${setConfig ''.SupportSettings.AboutLink = "https://mattermost.com"''}")
+    mutable.succeed("${setConfig ''.SupportSettings.HelpLink = "https://nixos.org/nixos/manual"''}")
+    mutable.systemctl("restart mattermost.service")
+    mutable.wait_for_open_port(8065)
+
+    # AboutLink and HelpLink should be changed
+    mutable.succeed("${expectConfig ''.AboutLink == "https://mattermost.com" and .HelpLink == "https://nixos.org/nixos/manual"''}")
+
+    ## Mostly mutable node tests ##
+    mostlyMutable.wait_for_unit("mattermost.service")
+    mostlyMutable.wait_for_open_port(8065)
+
+    # Get the initial config
+    mostlyMutable.succeed("${expectConfig ''.AboutLink == "https://nixos.org"''}")
+
+    # Edit the config
+    mostlyMutable.succeed("${setConfig ''.SupportSettings.AboutLink = "https://mattermost.com"''}")
+    mostlyMutable.succeed("${setConfig ''.SupportSettings.HelpLink = "https://nixos.org/nixos/manual"''}")
+    mostlyMutable.systemctl("restart mattermost.service")
+    mostlyMutable.wait_for_open_port(8065)
+
+    # AboutLink should be overridden by NixOS configuration; HelpLink should be what we set above
+    mostlyMutable.succeed("${expectConfig ''.AboutLink == "https://nixos.org" and .HelpLink == "https://nixos.org/nixos/manual"''}")
+
+    ## Immutable node tests ##
+    immutable.wait_for_unit("mattermost.service")
+    immutable.wait_for_open_port(8065)
+
+    # Get the initial config
+    immutable.succeed("${expectConfig ''.AboutLink == "https://nixos.org" and .HelpLink == "https://search.nixos.org"''}")
+
+    # Edit the config
+    immutable.succeed("${setConfig ''.SupportSettings.AboutLink = "https://mattermost.com"''}")
+    immutable.succeed("${setConfig ''.SupportSettings.HelpLink = "https://nixos.org/nixos/manual"''}")
+    immutable.systemctl("restart mattermost.service")
+    immutable.wait_for_open_port(8065)
+
+    # Our edits should be ignored on restart
+    immutable.succeed("${expectConfig ''.AboutLink == "https://nixos.org" and .HelpLink == "https://search.nixos.org"''}")
+  '';
+})