diff --git a/src/Compilation.zig b/src/Compilation.zig
index 5925a80f083ee5a4214274f9216490ff4ab71ecb..760c8f8b110ea2c2b635fabb5b156ddea394276d 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -6864,8 +6864,11 @@ fn addCommonCCArgs(
                 },
             }
 
-            // Homebrew targets without LLVM support; use communities's preferred macros.
             switch (target.os.tag) {
+                // LLVM doesn't distinguish between Solaris and illumos, but the illumos GCC fork
+                // defines this macro.
+                .illumos => try argv.append("__illumos__"),
+                // Homebrew targets without LLVM support; use communities's preferred macros.
                 .@"3ds" => try argv.append("-D__3DS__"),
                 .vita => try argv.append("-D__vita__"),
                 else => {},