diff --git a/logstash-core/logstash-core.gemspec b/logstash-core/logstash-core.gemspec
index 19608a8a812..d9ca01facd2 100644
--- a/logstash-core/logstash-core.gemspec
+++ b/logstash-core/logstash-core.gemspec
@@ -78,6 +78,7 @@ Gem::Specification.new do |gem|
 
   gem.add_runtime_dependency "jrjackson", "= #{ALL_VERSIONS.fetch('jrjackson')}" #(Apache 2.0 license)
 
+  gem.add_runtime_dependency "multi_json", "~> 1.19.1" # pinned until concurrent-ruby pin is lifted, multi_json 1.20.0-java requires concurrent-ruby ~> 1.2
   gem.add_runtime_dependency "elasticsearch", '~> 8'
   gem.add_runtime_dependency "manticore", '~> 0.6'