diff --git a/nixos/doc/manual/man-nixos-install.xml b/nixos/doc/manual/man-nixos-install.xml
index b205e23096875..12d0159719999 100644
--- a/nixos/doc/manual/man-nixos-install.xml
+++ b/nixos/doc/manual/man-nixos-install.xml
@@ -49,6 +49,12 @@
     <option>--flake</option> <replaceable>flake-uri</replaceable>
    </arg>
 
+   <arg>
+    <group choice='req'>
+     <arg choice='plain'><option>--impure</option></arg>
+    </group>
+   </arg>
+
    <arg>
      <arg choice='plain'>
        <option>--channel</option>
diff --git a/nixos/modules/installer/tools/nixos-install.sh b/nixos/modules/installer/tools/nixos-install.sh
index 90555f1458036..9279f11b4f325 100644
--- a/nixos/modules/installer/tools/nixos-install.sh
+++ b/nixos/modules/installer/tools/nixos-install.sh
@@ -64,7 +64,7 @@ while [ "$#" -gt 0 ]; do
         --no-bootloader)
             noBootLoader=1
             ;;
-        --show-trace)
+        --show-trace|--impure)
             extraBuildFlags+=("$i")
             ;;
         --help)