diff --git a/sentry-ruby/lib/sentry/transport/dummy_transport.rb b/sentry-ruby/lib/sentry/transport/dummy_transport.rb
index 68df2d122..c83dece62 100644
--- a/sentry-ruby/lib/sentry/transport/dummy_transport.rb
+++ b/sentry-ruby/lib/sentry/transport/dummy_transport.rb
@@ -8,22 +8,25 @@ def initialize(*)
       super
       @events = []
       @envelopes = []
+      @mutex = Mutex.new
     end
 
     def send_event(event)
-      @events << event
+      @mutex.synchronize { @events << event }
       super
     end
 
     def send_envelope(envelope)
-      @envelopes << envelope
+      @mutex.synchronize { @envelopes << envelope }
     end
 
     # Empties the captured events and envelopes so `TestHelper.clear_sentry_events`
     # also clears the dummy transport instance
     def clear
-      @events.clear
-      @envelopes.clear
+      @mutex.synchronize do
+        @events.clear
+        @envelopes.clear
+      end
     end
   end
 end
diff --git a/sentry-ruby/spec/sentry/transport/dummy_transport_spec.rb b/sentry-ruby/spec/sentry/transport/dummy_transport_spec.rb
new file mode 100644
index 000000000..235f999e8
--- /dev/null
+++ b/sentry-ruby/spec/sentry/transport/dummy_transport_spec.rb
@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+RSpec.describe Sentry::DummyTransport do
+  let(:configuration) do
+    Sentry::Configuration.new.tap do |config|
+      config.sdk_logger = Logger.new(nil)
+    end
+  end
+
+  subject(:transport) { described_class.new(configuration) }
+
+  it "keeps every envelope sent from concurrent threads" do
+    threads = Array.new(8) do
+      Thread.new { 500.times { transport.send_envelope(Sentry::Envelope.new) } }
+    end
+    threads.each(&:join)
+
+    expect(transport.envelopes.size).to eq(4000)
+    expect(transport.envelopes).to all(be_a(Sentry::Envelope))
+  end
+end