diff --git a/textproc/nlohmann-json/Makefile b/textproc/nlohmann-json/Makefile
index c6165d8553e..bba2883996e 100644
--- a/textproc/nlohmann-json/Makefile
+++ b/textproc/nlohmann-json/Makefile
@@ -1,10 +1,9 @@
 COMMENT =	JSON for modern C++
 
 # the "release" contain only json.hpp, and we want tests and CMake config file
-V =		3.11.2
+V =		3.12.0
 DIST_TUPLE =	github	nlohmann	json	v${V}	.
 PKGNAME =	nlohmann-json-${V}
-REVISION =	0
 
 TEST_DATA_DIR =	tests/test_data
 DIST_TUPLE +=	github	nlohmann	json_test_data	v3.1.0	${TEST_DATA_DIR}
@@ -29,6 +28,14 @@ NO_BUILD =		Yes
 pre-test:
 	@${MODCMAKE_BUILD_TARGET}
 
+# One test fails due to lack of LC_NUMERIC support in OpenBSD:
+# 49: /usr/ports/pobj/nlohmann-json-3.12.0/json-3.12.0/tests/src/unit-locale-cpp.cpp:134:
+# 49: TEST CASE:  locale-dependent test (LC_NUMERIC=de_DE)
+# 49:   check if locale is properly set
+# 49:
+# 49: /usr/ports/pobj/nlohmann-json-3.12.0/json-3.12.0/tests/src/unit-locale-cpp.cpp:142: ERROR: CHECK( std::string(buffer.data()) == "12,34" ) is NOT correct!
+# 49:   values: CHECK( 12.34 == 12,34 )
+
 # overwrite target, devel/cmake MODULE cannot pass ctest(1) arguments
 do-test:
 	# skip known to tail tests
diff --git a/textproc/nlohmann-json/distinfo b/textproc/nlohmann-json/distinfo
index 1526e41c2ec..2cec33693d3 100644
--- a/textproc/nlohmann-json/distinfo
+++ b/textproc/nlohmann-json/distinfo
@@ -1,4 +1,4 @@
-SHA256 (nlohmann-json-v3.11.2.tar.gz) = 1p+d62p14lgEZcbExREbicTcL6lOOoX80v/NmhQ9knM=
+SHA256 (nlohmann-json-v3.12.0.tar.gz) = S5LrDAbRBoP3RHzpQGy5fNS0U74Y1yeTIPey8CXBAYc=
 SHA256 (nlohmann-json_test_data-v3.1.0.tar.gz) = iEseIfOM/WpiwVnxt+Co9RaK5dqqOE7U3AwPpmYPIb0=
-SIZE (nlohmann-json-v3.11.2.tar.gz) = 8097673
+SIZE (nlohmann-json-v3.12.0.tar.gz) = 9678593
 SIZE (nlohmann-json_test_data-v3.1.0.tar.gz) = 115036393
diff --git a/textproc/nlohmann-json/patches/patch-include_nlohmann_detail_conversions_from_json_hpp b/textproc/nlohmann-json/patches/patch-include_nlohmann_detail_conversions_from_json_hpp
new file mode 100644
index 00000000000..349fa431a58
--- /dev/null
+++ b/textproc/nlohmann-json/patches/patch-include_nlohmann_detail_conversions_from_json_hpp
@@ -0,0 +1,17 @@
+https://github.com/nlohmann/json/pull/4736/commits/34868f90149de02432ea758a29227a6ad74f098c
+
+Index: include/nlohmann/detail/conversions/from_json.hpp
+--- include/nlohmann/detail/conversions/from_json.hpp.orig
++++ include/nlohmann/detail/conversions/from_json.hpp
+@@ -540,7 +540,10 @@ inline void from_json(const BasicJsonType& j, std_fs::
+         JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
+     }
+     const auto& s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
+-#ifdef JSON_HAS_CPP_20
++    // Checking for C++20 standard or later can be insufficient in case the
++    // library support for char8_t is either incomplete or was disabled
++    // altogether. Use the __cpp_lib_char8_t feature test instead.
++#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
+     p = std_fs::path(std::u8string_view(reinterpret_cast<const char8_t*>(s.data()), s.size()));
+ #else
+     p = std_fs::u8path(s); // accepts UTF-8 encoded std::string in C++17, deprecated in C++20
diff --git a/textproc/nlohmann-json/patches/patch-include_nlohmann_detail_conversions_to_json_hpp b/textproc/nlohmann-json/patches/patch-include_nlohmann_detail_conversions_to_json_hpp
new file mode 100644
index 00000000000..bfafe23ec44
--- /dev/null
+++ b/textproc/nlohmann-json/patches/patch-include_nlohmann_detail_conversions_to_json_hpp
@@ -0,0 +1,43 @@
+https://github.com/nlohmann/json/pull/4736/commits/34868f90149de02432ea758a29227a6ad74f098c
+
+Index: include/nlohmann/detail/conversions/to_json.hpp
+--- include/nlohmann/detail/conversions/to_json.hpp.orig
++++ include/nlohmann/detail/conversions/to_json.hpp
+@@ -15,7 +15,8 @@
+ 
+ #include <algorithm> // copy
+ #include <iterator> // begin, end
+-#include <string> // string
++#include <memory> // allocator_traits
++#include <string> //  basic_string, char_traits
+ #include <tuple> // tuple, get
+ #include <type_traits> // is_same, is_constructible, is_floating_point, is_enum, underlying_type
+ #include <utility> // move, forward, declval, pair
+@@ -440,15 +441,21 @@ inline void to_json(BasicJsonType& j, const T& t)
+ }
+ 
+ #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
++#if defined(__cpp_lib_char8_t)
++template<typename BasicJsonType, typename Tr, typename Allocator>
++inline void to_json(BasicJsonType& j, const std::basic_string<char8_t, Tr, Allocator>& s)
++{
++    using OtherAllocator = typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
++    j = std::basic_string<char, std::char_traits<char>, OtherAllocator>(s.begin(), s.end(), s.get_allocator());
++}
++#endif
++
+ template<typename BasicJsonType>
+ inline void to_json(BasicJsonType& j, const std_fs::path& p)
+ {
+-#ifdef JSON_HAS_CPP_20
+-    const std::u8string s = p.u8string();
+-    j = std::string(s.begin(), s.end());
+-#else
+-    j = p.u8string(); // returns std::string in C++17
+-#endif
++    // Returns either a std::string or a std::u8string depending whether library
++    // support for char8_t is enabled.
++    j = p.u8string();
+ }
+ #endif
+ 
diff --git a/textproc/nlohmann-json/patches/patch-single_include_nlohmann_json_hpp b/textproc/nlohmann-json/patches/patch-single_include_nlohmann_json_hpp
new file mode 100644
index 00000000000..c370f1fa9ad
--- /dev/null
+++ b/textproc/nlohmann-json/patches/patch-single_include_nlohmann_json_hpp
@@ -0,0 +1,55 @@
+https://github.com/nlohmann/json/pull/4736/commits/34868f90149de02432ea758a29227a6ad74f098c
+
+Index: single_include/nlohmann/json.hpp
+--- single_include/nlohmann/json.hpp.orig
++++ single_include/nlohmann/json.hpp
+@@ -5325,7 +5325,10 @@ inline void from_json(const BasicJsonType& j, std_fs::
+         JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
+     }
+     const auto& s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
+-#ifdef JSON_HAS_CPP_20
++    // Checking for C++20 standard or later can be insufficient in case the
++    // library support for char8_t is either incomplete or was disabled
++    // altogether. Use the __cpp_lib_char8_t feature test instead.
++#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
+     p = std_fs::path(std::u8string_view(reinterpret_cast<const char8_t*>(s.data()), s.size()));
+ #else
+     p = std_fs::u8path(s); // accepts UTF-8 encoded std::string in C++17, deprecated in C++20
+@@ -5380,7 +5383,8 @@ NLOHMANN_JSON_NAMESPACE_END
+ 
+ #include <algorithm> // copy
+ #include <iterator> // begin, end
+-#include <string> // string
++#include <memory> // allocator_traits
++#include <string> //  basic_string, char_traits
+ #include <tuple> // tuple, get
+ #include <type_traits> // is_same, is_constructible, is_floating_point, is_enum, underlying_type
+ #include <utility> // move, forward, declval, pair
+@@ -6087,15 +6091,21 @@ inline void to_json(BasicJsonType& j, const T& t)
+ }
+ 
+ #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
++#if defined(__cpp_lib_char8_t)
++template<typename BasicJsonType, typename Tr, typename Allocator>
++inline void to_json(BasicJsonType& j, const std::basic_string<char8_t, Tr, Allocator>& s)
++{
++    using OtherAllocator = typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
++    j = std::basic_string<char, std::char_traits<char>, OtherAllocator>(s.begin(), s.end(), s.get_allocator());
++}
++#endif
++
+ template<typename BasicJsonType>
+ inline void to_json(BasicJsonType& j, const std_fs::path& p)
+ {
+-#ifdef JSON_HAS_CPP_20
+-    const std::u8string s = p.u8string();
+-    j = std::string(s.begin(), s.end());
+-#else
+-    j = p.u8string(); // returns std::string in C++17
+-#endif
++    // Returns either a std::string or a std::u8string depending whether library
++    // support for char8_t is enabled.
++    j = p.u8string();
+ }
+ #endif
+ 
diff --git a/textproc/nlohmann-json/patches/patch-tests_src_unit-deserialization_cpp b/textproc/nlohmann-json/patches/patch-tests_src_unit-deserialization_cpp
new file mode 100644
index 00000000000..039f2639b71
--- /dev/null
+++ b/textproc/nlohmann-json/patches/patch-tests_src_unit-deserialization_cpp
@@ -0,0 +1,18 @@
+https://github.com/nlohmann/json/pull/4736/commits/34868f90149de02432ea758a29227a6ad74f098c
+
+Index: tests/src/unit-deserialization.cpp
+--- tests/src/unit-deserialization.cpp.orig
++++ tests/src/unit-deserialization.cpp
+@@ -1134,9 +1134,10 @@ TEST_CASE("deserialization")
+     }
+ }
+ 
+-// select the types to test - char8_t is only available in C++20
++// select the types to test - char8_t is only available since C++20 if and only
++// if __cpp_char8_t is defined.
+ #define TYPE_LIST(...) __VA_ARGS__
+-#ifdef JSON_HAS_CPP_20
++#if defined(__cpp_char8_t) && (__cpp_char8_t >= 201811L)
+     #define ASCII_TYPES TYPE_LIST(char, wchar_t, char16_t, char32_t, char8_t)
+ #else
+     #define ASCII_TYPES TYPE_LIST(char, wchar_t, char16_t, char32_t)
diff --git a/textproc/nlohmann-json/pkg/PLIST b/textproc/nlohmann-json/pkg/PLIST
index 9b7c95a3fbb..daa90621cc9 100644
--- a/textproc/nlohmann-json/pkg/PLIST
+++ b/textproc/nlohmann-json/pkg/PLIST
@@ -23,6 +23,7 @@ include/nlohmann/detail/iterators/iteration_proxy.hpp
 include/nlohmann/detail/iterators/iterator_traits.hpp
 include/nlohmann/detail/iterators/json_reverse_iterator.hpp
 include/nlohmann/detail/iterators/primitive_iterator.hpp
+include/nlohmann/detail/json_custom_base_class.hpp
 include/nlohmann/detail/json_pointer.hpp
 include/nlohmann/detail/json_ref.hpp
 include/nlohmann/detail/macro_scope.hpp
@@ -44,6 +45,7 @@ include/nlohmann/detail/output/output_adapters.hpp
 include/nlohmann/detail/output/serializer.hpp
 include/nlohmann/detail/string_concat.hpp
 include/nlohmann/detail/string_escape.hpp
+include/nlohmann/detail/string_utils.hpp
 include/nlohmann/detail/value_t.hpp
 include/nlohmann/json.hpp
 include/nlohmann/json_fwd.hpp