diff --git a/api/lib/opentelemetry/context/propagation/rack_env_getter.rb b/api/lib/opentelemetry/context/propagation/rack_env_getter.rb
index 18adef4b88..27e83fd736 100644
--- a/api/lib/opentelemetry/context/propagation/rack_env_getter.rb
+++ b/api/lib/opentelemetry/context/propagation/rack_env_getter.rb
@@ -32,7 +32,8 @@ def keys(carrier)
         private
 
         def to_rack_key(key)
-          ret = 'HTTP_' + key # rubocop:disable Style/StringConcatenation
+          # Use + for mutable string interpolation in pre-Ruby 3.0.
+          ret = +"HTTP_#{key}"
           ret.tr!('-', '_')
           ret.upcase!
           ret
diff --git a/common/lib/opentelemetry/common/propagation/rack_env_getter.rb b/common/lib/opentelemetry/common/propagation/rack_env_getter.rb
index 7c97082e80..d51d84c66a 100644
--- a/common/lib/opentelemetry/common/propagation/rack_env_getter.rb
+++ b/common/lib/opentelemetry/common/propagation/rack_env_getter.rb
@@ -29,7 +29,8 @@ def keys(carrier)
         private
 
         def to_rack_key(key)
-          ret = "HTTP_#{key}"
+          # Use + for mutable string interpolation in pre-Ruby 3.0.
+          ret = +"HTTP_#{key}"
           ret.tr!('-', '_')
           ret.upcase!
           ret
diff --git a/propagator/jaeger/test/text_map_propagator_test.rb b/propagator/jaeger/test/text_map_propagator_test.rb
index 0f9320aba9..cd2e0d1eee 100644
--- a/propagator/jaeger/test/text_map_propagator_test.rb
+++ b/propagator/jaeger/test/text_map_propagator_test.rb
@@ -265,7 +265,8 @@ def create_context(trace_id:,
     it 'injects to rack keys' do
       rack_env_setter = Object.new
       def rack_env_setter.set(carrier, key, value)
-        rack_key = 'HTTP_' + key
+        # Use + for mutable string interpolation in pre-Ruby 3.0.
+        rack_key = +"HTTP_#{key}"
         rack_key.tr!('-', '_')
         rack_key.upcase!
         carrier[rack_key] = value