diff --git a/cpp/src/arrow/compute/kernels/codegen_internal.h b/cpp/src/arrow/compute/kernels/codegen_internal.h
index 90c4b32415aed..8c49e796623e7 100644
--- a/cpp/src/arrow/compute/kernels/codegen_internal.h
+++ b/cpp/src/arrow/compute/kernels/codegen_internal.h
@@ -663,14 +663,16 @@ struct ScalarUnaryNotNullStateful {
     static void Exec(const ThisType& functor, KernelContext* ctx, const ArrayData& arg0,
                      Datum* out) {
       ArrayData* out_arr = out->mutable_array();
-      auto out_data = out_arr->GetMutableValues<uint8_t>(1, 0) + 16 * out_arr->offset;
+      // Decimal128 data buffers are not safely reinterpret_cast-able on big-endian
+      using endian_agnostic = std::array<uint8_t, sizeof(Decimal128)>;
+      auto out_data = out_arr->GetMutableValues<endian_agnostic>(1);
       VisitArrayValuesInline<Arg0Type>(
           arg0,
           [&](Arg0Value v) {
-            functor.op.template Call<OutValue, Arg0Value>(ctx, v).ToBytes(out_data);
-            out_data += 16;
+            functor.op.template Call<OutValue, Arg0Value>(ctx, v).ToBytes(
+                out_data++->data());
           },
-          [&]() { out_data += 16; });
+          [&]() { ++out_data; });
     }
   };