diff --git a/builder/build.go b/builder/build.go
index 44f41eb233..537fe8846f 100644
--- a/builder/build.go
+++ b/builder/build.go
@@ -396,6 +396,7 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 				if errs != nil {
 					return newMultiError(errs, pkg.ImportPath)
 				}
+				transform.StripNoneParamAttrs(mod)
 				if err := llvm.VerifyModule(mod, llvm.PrintMessageAction); err != nil {
 					return errors.New("verification error after compiling package " + pkg.ImportPath)
 				}
@@ -481,6 +482,7 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 				if err != nil {
 					return err
 				}
+				transform.StripNoneParamAttrs(mod)
 				if err := llvm.VerifyModule(mod, llvm.PrintMessageAction); err != nil {
 					return errors.New("verification error after interpreting " + pkgInit.Name())
 				}
@@ -1191,6 +1193,7 @@ func optimizeProgram(mod llvm.Module, config *compileopts.Config) error {
 		// linked IR is _expensive_ because dead code hasn't been removed yet,
 		// easily costing a few hundred milliseconds. Therefore, only do it when
 		// specifically requested.
+		transform.StripNoneParamAttrs(mod)
 		if err := llvm.VerifyModule(mod, llvm.PrintMessageAction); err != nil {
 			return errors.New("verification error after interpreting runtime.initAll")
 		}
@@ -1202,6 +1205,7 @@ func optimizeProgram(mod llvm.Module, config *compileopts.Config) error {
 	if len(errs) > 0 {
 		return newMultiError(errs, "")
 	}
+	transform.StripNoneParamAttrs(mod)
 	if err := llvm.VerifyModule(mod, llvm.PrintMessageAction); err != nil {
 		return errors.New("verification failure after LLVM optimization passes")
 	}
@@ -1553,3 +1557,4 @@ func b2u8(b bool) uint8 {
 	}
 	return 0
 }
+
diff --git a/cgo/libclang_config_llvm20.go b/cgo/libclang_config_llvm20.go
index faa2e54d55..14902a2957 100644
--- a/cgo/libclang_config_llvm20.go
+++ b/cgo/libclang_config_llvm20.go
@@ -1,4 +1,4 @@
-//go:build !byollvm && !llvm15 && !llvm16 && !llvm17 && !llvm18 && !llvm19
+//go:build !byollvm && llvm20
 
 package cgo
 
diff --git a/cgo/libclang_config_llvm21.go b/cgo/libclang_config_llvm21.go
new file mode 100644
index 0000000000..b26a69e6f2
--- /dev/null
+++ b/cgo/libclang_config_llvm21.go
@@ -0,0 +1,15 @@
+//go:build !byollvm && !llvm15 && !llvm16 && !llvm17 && !llvm18 && !llvm19 && !llvm20
+
+package cgo
+
+/*
+#cgo linux        CFLAGS:  -I/usr/include/llvm-21 -I/usr/include/llvm-c-21 -I/usr/lib/llvm-21/include
+#cgo darwin,amd64 CFLAGS:  -I/usr/local/opt/llvm@21/include
+#cgo darwin,arm64 CFLAGS:  -I/opt/homebrew/opt/llvm@21/include
+#cgo freebsd      CFLAGS:  -I/usr/local/llvm21/include
+#cgo linux        LDFLAGS: -L/usr/lib/llvm-21/lib -lclang
+#cgo darwin,amd64 LDFLAGS: -L/usr/local/opt/llvm@21/lib -lclang
+#cgo darwin,arm64 LDFLAGS: -L/opt/homebrew/opt/llvm@21/lib -lclang
+#cgo freebsd      LDFLAGS: -L/usr/local/llvm21/lib -lclang
+*/
+import "C"
diff --git a/go.mod b/go.mod
index 7d4ea5f189..40b741aca7 100644
--- a/go.mod
+++ b/go.mod
@@ -18,7 +18,7 @@ require (
 	golang.org/x/sys v0.30.0
 	golang.org/x/tools v0.30.0
 	gopkg.in/yaml.v2 v2.4.0
-	tinygo.org/x/go-llvm v0.0.0-20250422114502-b8f170971e74
+	tinygo.org/x/go-llvm v0.0.0-20250929104024-00fb4309ddd2
 )
 
 require (
diff --git a/go.sum b/go.sum
index 8c2330c3c5..5d6bb75689 100644
--- a/go.sum
+++ b/go.sum
@@ -60,3 +60,5 @@ gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 tinygo.org/x/go-llvm v0.0.0-20250422114502-b8f170971e74 h1:ovavgTdIBWCH8YWlcfq9gkpoyT1+IxMKSn+Df27QwE8=
 tinygo.org/x/go-llvm v0.0.0-20250422114502-b8f170971e74/go.mod h1:GFbusT2VTA4I+l4j80b17KFK+6whv69Wtny5U+T8RR0=
+tinygo.org/x/go-llvm v0.0.0-20250929104024-00fb4309ddd2 h1:PsrNpgfZNjozTahZwNrsRfdUxYPATE60vUn6UjK+/Jo=
+tinygo.org/x/go-llvm v0.0.0-20250929104024-00fb4309ddd2/go.mod h1:GFbusT2VTA4I+l4j80b17KFK+6whv69Wtny5U+T8RR0=
diff --git a/transform/optimizer.go b/transform/optimizer.go
index 54f9762bc4..2b34ee9c81 100644
--- a/transform/optimizer.go
+++ b/transform/optimizer.go
@@ -129,6 +129,7 @@ func Optimize(mod llvm.Module, config *compileopts.Config) []error {
 			return errs
 		}
 	}
+	StripNoneParamAttrs(mod)
 	if err := llvm.VerifyModule(mod, llvm.PrintMessageAction); err != nil {
 		return []error{errors.New("optimizations caused a verification failure")}
 	}
@@ -156,6 +157,7 @@ func Optimize(mod llvm.Module, config *compileopts.Config) []error {
 
 	hasGCPass := MakeGCStackSlots(mod)
 	if hasGCPass {
+		StripNoneParamAttrs(mod)
 		if err := llvm.VerifyModule(mod, llvm.PrintMessageAction); err != nil {
 			return []error{errors.New("GC pass caused a verification failure")}
 		}
@@ -172,3 +174,16 @@ var functionsUsedInTransforms = []string{
 	"runtime.free",
 	"runtime.nilPanic",
 }
+
+// StripNoneParamAttrs removes the "none" enum attribute (kind 0) from all
+// function parameters. LLVM 21 removed "none" as a valid parameter attribute.
+func StripNoneParamAttrs(mod llvm.Module) {
+	noneKind := llvm.AttributeKindID("none")
+	if noneKind == 0 {
+		for fn := mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
+			for i := 0; i < fn.ParamsCount(); i++ {
+				fn.RemoveEnumAttributeAtIndex(i+1, 0)
+			}
+		}
+	}
+}