diff --git a/modules/testing/test-instrumentation.nix b/modules/testing/test-instrumentation.nix
index 6f47a6a9c39..5ff7201acd3 100644
--- a/modules/testing/test-instrumentation.nix
+++ b/modules/testing/test-instrumentation.nix
@@ -77,6 +77,10 @@ in
     # Send all of /var/log/messages to the serial port.
     services.syslogd.extraConfig = "*.* /dev/ttyS0";
 
+    # Clear the kernel log buffer before starting klogd to prevent it
+    # from printing messages that we have already seen.
+    jobs.klogd.preStart = "dmesg -c > /dev/null";
+
     # Prevent tests from accessing the Internet.
     networking.defaultGateway = mkOverrideTemplate 150 {} "";
     networking.nameservers = mkOverrideTemplate 150 {} [ ];