diff --git a/numbast/src/numbast/callconv.py b/numbast/src/numbast/callconv.py
index 5e10eadd..94df7b40 100644
--- a/numbast/src/numbast/callconv.py
+++ b/numbast/src/numbast/callconv.py
@@ -1,6 +1,7 @@
 # SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
+# NUMBAST_RETVAL_ALIGN_FIX_APPLIED
 from numbast.args import prepare_ir_types
 from numbast.intent import IntentPlan
 
@@ -107,6 +108,28 @@ def _lower_impl(self, builder, context, sig, args):
         else:
             retval_ty = context.get_value_type(cxx_return_type)
             retval_ptr = builder.alloca(retval_ty, name="retval")
+            # Use the Numba type's alignof_ to set the alloca alignment.
+            #
+            # LLVM computes struct ABI alignment as the max alignment of its
+            # members.  For CUDA vector types (float2, float4, uchar4, …)
+            # declared with __align__(N) in the CUDA headers, N can exceed
+            # the member alignment: float2 is {float,float} with member
+            # alignment 4 B but __align__(8).  LLVM therefore assigns a 4 B
+            # alloca, while the NVRTC shim uses a vector instruction
+            # (ld/st.v2.f32) that requires 8 B alignment, causing
+            # cudaErrorMisalignedAddress at runtime.
+            #
+            # Numbast's struct binder already sets alignof_ on user-defined
+            # bound structs (propagated from ast_canopy).  For built-in CUDA
+            # vector types, callers must set alignof_ on the Numba type when
+            # registering it in CTYPE_MAPS.  When alignof_ is present it is
+            # used here, matching the convention already applied to loads and
+            # stores (getattr(argty, "alignof_", None)).  When absent, LLVM's
+            # default ABI alignment is used, which is correct for scalars and
+            # structs without an explicit __align__ attribute.
+            _nb_align = getattr(cxx_return_type, "alignof_", None)
+            if _nb_align is not None:
+                retval_ptr.align = _nb_align
 
         # 2. Prepare arguments
         if self._intent_plan is None:
@@ -154,8 +177,11 @@ def _lower_impl(self, builder, context, sig, args):
                     ptrs.append(arg)
                 else:
                     ptr = cgutils.alloca_once(builder, vty)
+                    _nb_align = getattr(argty, "alignof_", None)
+                    if _nb_align is not None:
+                        ptr.align = _nb_align  # see retval_ptr comment above
                     builder.store(
-                        arg, ptr, align=getattr(argty, "alignof_", None)
+                        arg, ptr, align=_nb_align
                     )
                     ptrs.append(ptr)
         else:
@@ -175,6 +201,9 @@ def _lower_impl(self, builder, context, sig, args):
                     out_nbty = self._out_return_types[out_pos]
                     vty = context.get_value_type(out_nbty)
                     ptr = cgutils.alloca_once(builder, vty)
+                    _nb_align = getattr(out_nbty, "alignof_", None)
+                    if _nb_align is not None:
+                        ptr.align = _nb_align  # see retval_ptr comment above
                     ptrs.append(ptr)
                     arg_pointer_types.append(ir.PointerType(vty))
                     out_return_ptrs.append((out_nbty, ptr))
@@ -194,8 +223,11 @@ def _lower_impl(self, builder, context, sig, args):
                     arg_pointer_types.append(vty)
                 else:
                     ptr = cgutils.alloca_once(builder, vty)
+                    _nb_align = getattr(argty, "alignof_", None)
+                    if _nb_align is not None:
+                        ptr.align = _nb_align  # see retval_ptr comment above
                     builder.store(
-                        arg, ptr, align=getattr(argty, "alignof_", None)
+                        arg, ptr, align=_nb_align
                     )
                     ptrs.append(ptr)
                     arg_pointer_types.append(ir.PointerType(vty))