diff --git a/pkgs/development/interpreters/spidermonkey/78.nix b/pkgs/development/interpreters/spidermonkey/78.nix
index a534026d9ecd..4f525c8f0a5b 100644
--- a/pkgs/development/interpreters/spidermonkey/78.nix
+++ b/pkgs/development/interpreters/spidermonkey/78.nix
@@ -107,6 +107,10 @@ stdenv.mkDerivation rec {
 
   depsBuildBuild = [ buildPackages.stdenv.cc ];
 
+  # cc-rs insists on using -mabi=lp64 (soft-float) for riscv64,
+  # while we have a double-float toolchain
+  NIX_CFLAGS_COMPILE = lib.optionalString (with stdenv.hostPlatform; isRiscV && is64bit) "-mabi=lp64d";
+
   # Remove unnecessary static lib
   preFixup = ''
     moveToOutput bin/js78-config "$dev"