diff --git a/pkgs/development/haskell-modules/generic-builder.nix b/pkgs/development/haskell-modules/generic-builder.nix
index 5cebe4aec5d0..4ff85fe5d540 100644
--- a/pkgs/development/haskell-modules/generic-builder.nix
+++ b/pkgs/development/haskell-modules/generic-builder.nix
@@ -805,6 +805,7 @@ lib.fix (
             ${optionalString doHoogle "--hoogle"} \
             ${optionalString doHaddockQuickjump "--quickjump"} \
             ${optionalString (isLibrary && hyperlinkSource) "--hyperlink-source"} \
+            ${optionalString enableParallelBuilding "--haddock-option=-j$NIX_BUILD_CORES"} \
             ${lib.concatStringsSep " " haddockFlags}
         ''}
         runHook postHaddock