Romulan Security, also known as the Romulan security forces, was an agency of the Romulan Star Empire charged with addressing potential threats to the Empire. This included seeking out and arresting potential spies.
In 2368, Senator Pardek told Jean-Luc Picard, who was on an undercover mission to Romulus, that Romulan Security was alerted to his presence on their planet. He added that it was fortunate that Pardek found him first. (TNG: "Unification I")