A subspace monitor was a type of tranceiver tuned to keep track of subspace signals and communication.

In 2254, Admiral Comsol contacted the USS Enterprise informing them that subspace monitors had detected transmissions received aboard the Enterprise replaying Pike's mission to Talos IV, were in fact coming from Talos IV, in violation of Starfleet General Order 7. (TOS: "The Menagerie, Part I")