diff --git a/pkgs/development/compilers/llvm/18/llvm/backport-darwin-triple-parsing.patch b/pkgs/development/compilers/llvm/18/llvm/backport-darwin-triple-parsing.patch
new file mode 100644
index 000000000000..9426f99512b7
--- /dev/null
+++ b/pkgs/development/compilers/llvm/18/llvm/backport-darwin-triple-parsing.patch
@@ -0,0 +1,34 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Randy Eckenrode <randy@largeandhighquality.com>
+Date: Tue, 11 Aug 2026 20:12:42 -0400
+Subject: [PATCH] backport-darwin-triple-parsing
+
+---
+ lib/TargetParser/Triple.cpp | 10 ++++++++--
+ 1 file changed, 8 insertions(+), 2 deletions(-)
+
+diff --git a/lib/TargetParser/Triple.cpp b/lib/TargetParser/Triple.cpp
+index 0bbe8a3cedfd..4143c3eac05d 100644
+--- a/lib/TargetParser/Triple.cpp
++++ b/lib/TargetParser/Triple.cpp
+@@ -1253,9 +1253,15 @@ bool Triple::getMacOSXVersion(VersionTuple &Version) const {
+     }
+     if (Version.getMajor() <= 19) {
+       Version = VersionTuple(10, Version.getMajor() - 4);
+-    } else {
+-      // darwin20+ corresponds to macOS 11+.
++    } else if (Version.getMajor() < 25) {
++      // darwin20-24 corresponds to macOS 11-15.
+       Version = VersionTuple(11 + Version.getMajor() - 20);
++    } else if ((Version.getMajor() == 25) || (Version.getMajor() == 26)) {
++      // darwin25-26 corresponds to macOS 26-27.
++      Version = VersionTuple(Version.getMajor() + 1);
++    } else {
++      // Starting with darwin27, it naturally corresponds to the same macOS
++      // version.
+     }
+     break;
+   case MacOSX:
+-- 
+2.54.0
+
diff --git a/pkgs/development/compilers/llvm/21/llvm/backport-darwin-triple-parsing.patch b/pkgs/development/compilers/llvm/21/llvm/backport-darwin-triple-parsing.patch
new file mode 100644
index 000000000000..7f2cc06b51d4
--- /dev/null
+++ b/pkgs/development/compilers/llvm/21/llvm/backport-darwin-triple-parsing.patch
@@ -0,0 +1,31 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Randy Eckenrode <randy@largeandhighquality.com>
+Date: Tue, 11 Aug 2026 20:19:29 -0400
+Subject: [PATCH] backport-darwin-triple-parsing
+
+---
+ lib/TargetParser/Triple.cpp | 7 +++++--
+ 1 file changed, 5 insertions(+), 2 deletions(-)
+
+diff --git a/lib/TargetParser/Triple.cpp b/lib/TargetParser/Triple.cpp
+index 0584c941d2e6..30cf78dbaabc 100644
+--- a/lib/TargetParser/Triple.cpp
++++ b/lib/TargetParser/Triple.cpp
+@@ -1449,9 +1449,12 @@ bool Triple::getMacOSXVersion(VersionTuple &Version) const {
+     } else if (Version.getMajor() < 25) {
+       // darwin20-24 corresponds to macOS 11-15.
+       Version = VersionTuple(11 + Version.getMajor() - 20);
+-    } else {
+-      // darwin25 corresponds with macOS26+.
++    } else if ((Version.getMajor() == 25) || (Version.getMajor() == 26)) {
++      // darwin25-26 corresponds to macOS 26-27.
+       Version = VersionTuple(Version.getMajor() + 1);
++    } else {
++      // Starting with darwin27, it naturally corresponds to the same macOS
++      // version.
+     }
+     break;
+   case MacOSX:
+-- 
+2.54.0
+
diff --git a/pkgs/development/compilers/llvm/common/llvm/default.nix b/pkgs/development/compilers/llvm/common/llvm/default.nix
index d5dcd4d58cce..222f7fb6e54d 100644
--- a/pkgs/development/compilers/llvm/common/llvm/default.nix
+++ b/pkgs/development/compilers/llvm/common/llvm/default.nix
@@ -222,7 +222,14 @@ stdenv.mkDerivation (
               hash = "sha256-3hkbYPUVRAtWpo5qBmc2jLZLivURMx8T0GQomvNZesc=";
               stripLen = 1;
             }
-          );
+          )
+      ++ lib.optionals (lib.versionOlder release_version "23") [
+        # As of macOS 27 (and iOS 27, etc), the Darwin version number is the same as the OS version number.
+        # This change breaks target parsing because `darwin27` is incorrectly interpreted as macOS 28.
+        # This patch is a backport of the target parsing changes in LLVM 23, which fixes the problem.
+        # Hopefully, Apple does not change the version number scheme again any time soon.
+        (getVersionFile "llvm/backport-darwin-triple-parsing.patch")
+      ];
 
     nativeBuildInputs = [
       cmake
diff --git a/pkgs/development/compilers/llvm/common/patches.nix b/pkgs/development/compilers/llvm/common/patches.nix
index 76ae90c3aaa7..5ea9faa3d2e2 100644
--- a/pkgs/development/compilers/llvm/common/patches.nix
+++ b/pkgs/development/compilers/llvm/common/patches.nix
@@ -20,6 +20,17 @@
       path = ../18;
     }
   ];
+  "llvm/backport-darwin-triple-parsing.patch" = [
+    {
+      after = "18";
+      before = "21";
+      path = ../18;
+    }
+    {
+      after = "21";
+      path = ../21;
+    }
+  ];
   "llvm/gnu-install-dirs.patch" = [
     {
       after = "23";