diff --git a/include/boost/smart_ptr/detail/sp_type_traits.hpp b/include/boost/smart_ptr/detail/sp_type_traits.hpp
index 1437c7b..6e0dcee 100644
--- a/include/boost/smart_ptr/detail/sp_type_traits.hpp
+++ b/include/boost/smart_ptr/detail/sp_type_traits.hpp
@@ -22,6 +22,16 @@ template<class T, std::size_t N> struct sp_is_bounded_array< T[N] >: std::true_t
 {
 };
 
+// std::is_unbounded_array (C++20)
+
+template<class T> struct sp_is_unbounded_array: std::false_type
+{
+};
+
+template<class T> struct sp_is_unbounded_array< T[] >: std::true_type
+{
+};
+
 } // namespace detail
 } // namespace boost
 
diff --git a/test/Jamfile b/test/Jamfile
index 3de1ebc..8c67901 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -426,3 +426,4 @@ run sp_unique_ptr_test2.cpp ;
 run sp_move_only_deleter.cpp ;
 
 run sp_is_bounded_array_test.cpp ;
+run sp_is_unbounded_array_test.cpp ;
diff --git a/test/sp_is_unbounded_array_test.cpp b/test/sp_is_unbounded_array_test.cpp
new file mode 100644
index 0000000..f929e4e
--- /dev/null
+++ b/test/sp_is_unbounded_array_test.cpp
@@ -0,0 +1,27 @@
+// Copyright 2024 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/smart_ptr/detail/sp_type_traits.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+
+struct X;
+
+int main()
+{
+    using boost::detail::sp_is_unbounded_array;
+
+    BOOST_TEST_TRAIT_FALSE(( sp_is_unbounded_array<void> ));
+    BOOST_TEST_TRAIT_FALSE(( sp_is_unbounded_array<int> ));
+    BOOST_TEST_TRAIT_FALSE(( sp_is_unbounded_array<X> ));
+
+    BOOST_TEST_TRAIT_TRUE(( sp_is_unbounded_array<int[]> ));
+    BOOST_TEST_TRAIT_TRUE(( sp_is_unbounded_array<X[]> ));
+
+    BOOST_TEST_TRAIT_FALSE(( sp_is_unbounded_array<int[1]> ));
+    BOOST_TEST_TRAIT_FALSE(( sp_is_unbounded_array<int[7]> ));
+    BOOST_TEST_TRAIT_FALSE(( sp_is_unbounded_array<X[1]> ));
+    BOOST_TEST_TRAIT_FALSE(( sp_is_unbounded_array<X[7]> ));
+
+    return boost::report_errors();
+}