diff --git a/include/nlohmann/detail/hash.hpp b/include/nlohmann/detail/hash.hpp
index d3313e968..4094cc94f 100644
--- a/include/nlohmann/detail/hash.hpp
+++ b/include/nlohmann/detail/hash.hpp
@@ -9,7 +9,7 @@ namespace detail
 {
 
 // boost::hash_combine
-std::size_t combine(std::size_t seed, std::size_t h) noexcept
+inline std::size_t combine(std::size_t seed, std::size_t h) noexcept
 {
     seed ^= h + 0x9e3779b9 + (seed << 6U) + (seed >> 2U);
     return seed;
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 1d8b5fb61..8c604c528 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -4454,7 +4454,7 @@ namespace detail
 {
 
 // boost::hash_combine
-std::size_t combine(std::size_t seed, std::size_t h) noexcept
+inline std::size_t combine(std::size_t seed, std::size_t h) noexcept
 {
     seed ^= h + 0x9e3779b9 + (seed << 6U) + (seed >> 2U);
     return seed;