diff --git a/Gemfile b/Gemfile
index 0884ad5b116..8c0e23dee70 100644
--- a/Gemfile
+++ b/Gemfile
@@ -75,6 +75,7 @@ group :test do
   gem 'cucumber', '< 12'
   gem 'multipart-post'
   gem 'opentelemetry-sdk'
+  gem 'ostruct' # Specs require it directly; Oj 3.17.8 removed its runtime dependency.
   gem 'rspec'
   gem 'webmock'
 end
diff --git a/gems/aws-sdk-core/spec/seahorse/client/h2/handler_spec.rb b/gems/aws-sdk-core/spec/seahorse/client/h2/handler_spec.rb
index fcfab4ac982..96311c00a74 100644
--- a/gems/aws-sdk-core/spec/seahorse/client/h2/handler_spec.rb
+++ b/gems/aws-sdk-core/spec/seahorse/client/h2/handler_spec.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require_relative '../../../spec_helper'
+require 'ostruct'
 
 module Seahorse
   module Client
diff --git a/gems/aws-sdk-core/spec/seahorse/client/net_http/handler_spec.rb b/gems/aws-sdk-core/spec/seahorse/client/net_http/handler_spec.rb
index 9c2f42a96a1..7694b2bd6c6 100644
--- a/gems/aws-sdk-core/spec/seahorse/client/net_http/handler_spec.rb
+++ b/gems/aws-sdk-core/spec/seahorse/client/net_http/handler_spec.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require_relative '../../../spec_helper'
+require 'ostruct'
 
 module Seahorse
   module Client
diff --git a/gems/aws-sdk-s3/spec/encryptionv3/decrypt_handler_spec.rb b/gems/aws-sdk-s3/spec/encryptionv3/decrypt_handler_spec.rb
index 2d193b7c25d..d0f42fac2ef 100644
--- a/gems/aws-sdk-s3/spec/encryptionv3/decrypt_handler_spec.rb
+++ b/gems/aws-sdk-s3/spec/encryptionv3/decrypt_handler_spec.rb
@@ -1,4 +1,5 @@
 require_relative '../spec_helper'
+require 'ostruct'
 require 'base64'
 require 'openssl'