diff --git a/base/test/launcher/teamcity_service_messages.cc b/base/test/launcher/teamcity_service_messages.cc
index 618b54634ac..b8eafe09ab8 100644
--- a/base/test/launcher/teamcity_service_messages.cc
+++ b/base/test/launcher/teamcity_service_messages.cc
@@ -53,18 +53,22 @@ class EscapedValue {
 TeamcityServiceMessages::Message::Message(std::ostream& ostream,
                                           std::string_view name)
     : ostream_(ostream) {
-  (*ostream_) << "##teamcity[" << name;
+  // Use stringstream to format the message before writing it to stdout.
+  sstream_ << "##teamcity[" << name;
 }
 
 TeamcityServiceMessages::Message::~Message() {
-  (*ostream_) << "]" << std::endl;
+  sstream_ << "]" << std::endl;
+  // Important: output into stdout in a single call to not mix with outputs from
+  // other threads.
+  (*ostream_) << sstream_.str() << std::flush;
 }
 
 TeamcityServiceMessages::Message&
 TeamcityServiceMessages::Message::WriteProperty(std::string_view name,
                                                 std::string_view value) {
   if (!value.empty()) {
-    (*ostream_) << " " << name << "='" << EscapedValue(value) << "'";
+    sstream_ << " " << name << "='" << EscapedValue(value) << "'";
   }
   return *this;
 }
diff --git a/base/test/launcher/teamcity_service_messages.h b/base/test/launcher/teamcity_service_messages.h
index 8107f1a4106..7713cb4a809 100644
--- a/base/test/launcher/teamcity_service_messages.h
+++ b/base/test/launcher/teamcity_service_messages.h
@@ -7,6 +7,7 @@
 #define BRAVE_BASE_TEST_LAUNCHER_TEAMCITY_SERVICE_MESSAGES_H_
 
 #include <ostream>
+#include <sstream>
 #include <string_view>
 
 #include "base/memory/raw_ref.h"
@@ -48,6 +49,7 @@ class TeamcityServiceMessages {
 
    private:
     raw_ref<std::ostream> ostream_;
+    std::ostringstream sstream_;
   };
 
   raw_ref<std::ostream> ostream_;
diff --git a/build/commands/lib/test.js b/build/commands/lib/test.js
index 1afb0f856d8..17bab943cc7 100644
--- a/build/commands/lib/test.js
+++ b/build/commands/lib/test.js
@@ -164,6 +164,10 @@ const runTests = (passthroughArgs, suite, buildConfig, options) => {
             `--avd-config tools/android/avd/proto/generic_android${options.android_test_emulator_version}.textpb`)
       }
       let runOptions = config.defaultOptions
+      if (config.isTeamcity) {
+        // Stdout and stderr must be separate for a test launcher.
+        runOptions.stdio = 'inherit'
+      }
       if (options.output)
         // When test results are saved to a file, callers (such as CI) generate
         // and analyze test reports as a next step. These callers are typically