diff --git a/pkgs/by-name/ou/outguess/package.nix b/pkgs/by-name/ou/outguess/package.nix
index 903c4754ffad..3d682afab665 100644
--- a/pkgs/by-name/ou/outguess/package.nix
+++ b/pkgs/by-name/ou/outguess/package.nix
@@ -22,6 +22,17 @@ stdenv.mkDerivation (finalAttrs: {
 
   configureFlags = [ "--with-generic-jconfig" ];
 
+  # Fix build with modern compilers (GCC 14+)
+  # The bundled jpeg-6b-steg library uses K&R-style function declarations
+  # that are incompatible with modern C standards
+  postPatch = ''
+    substituteInPlace src/jpeg-6b-steg/jmorecfg.h \
+      --replace-fail '#define JMETHOD(type,methodname,arglist)  type (*methodname) ()' \
+                     '#define JMETHOD(type,methodname,arglist)  type (*methodname) arglist'
+    substituteInPlace src/jpeg-6b-steg/jpeglib.h \
+      --replace-fail '#ifdef HAVE_PROTOTYPES' '#if 1  /* Force ANSI prototypes */'
+  '';
+
   meta = {
     description = "Universal steganographic tool that allows the insertion of hidden information into the redundant bits of data sources";
     homepage = "https://github.com/resurrecting-open-source-projects/outguess";