diff --git a/pkgs/applications/gis/qgis/unwrapped-ltr.nix b/pkgs/applications/gis/qgis/unwrapped-ltr.nix
index 39720283057f..5e35e56fe5fe 100644
--- a/pkgs/applications/gis/qgis/unwrapped-ltr.nix
+++ b/pkgs/applications/gis/qgis/unwrapped-ltr.nix
@@ -134,7 +134,11 @@ in mkDerivation rec {
     "-DWITH_3D=True"
     "-DWITH_PDAL=TRUE"
   ] ++ lib.optional (!withWebKit) "-DWITH_QTWEBKIT=OFF"
-    ++ lib.optional withGrass "-DGRASS_PREFIX7=${grass}/grass78";
+    ++ lib.optional withGrass (let
+        gmajor = lib.versions.major grass.version;
+        gminor = lib.versions.minor grass.version;
+      in "-DGRASS_PREFIX${gmajor}=${grass}/grass${gmajor}${gminor}"
+    );
 
   dontWrapGApps = true; # wrapper params passed below
 
diff --git a/pkgs/applications/gis/qgis/unwrapped.nix b/pkgs/applications/gis/qgis/unwrapped.nix
index d8af75375697..929a40d6daab 100644
--- a/pkgs/applications/gis/qgis/unwrapped.nix
+++ b/pkgs/applications/gis/qgis/unwrapped.nix
@@ -134,7 +134,11 @@ in mkDerivation rec {
     "-DWITH_3D=True"
     "-DWITH_PDAL=TRUE"
   ] ++ lib.optional (!withWebKit) "-DWITH_QTWEBKIT=OFF"
-    ++ lib.optional withGrass "-DGRASS_PREFIX7=${grass}/grass78";
+    ++ lib.optional withGrass (let
+        gmajor = lib.versions.major grass.version;
+        gminor = lib.versions.minor grass.version;
+      in "-DGRASS_PREFIX${gmajor}=${grass}/grass${gmajor}${gminor}"
+    );
 
   dontWrapGApps = true; # wrapper params passed below