This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
The Romulan Senate, or the Romulan Imperial Senate, was the main governing body of the Romulan Star Empire, seated in the Hall of State on the planet Romulus. The Senate was headed by the praetor, followed by the proconsul, the vice-proconsul, and finally the senators. The Senate did not sit on the third day of the Romulan week. It worked on a timetable that was difficult to predict. (TNG: "Unification I"; VOY: "Eye of the Needle")
After a combined Tal Shiar-Obsidian Order fleet entered the Gamma Quadrant on a mission to wipe out the Founders, the Tal Shiar sent a message to the Senate that explained its actions. The Senate released a statement explaining that it had no previous knowledge of the Tal Shiar's plans and claimed it was exploring options to stop them. (DS9: "The Die is Cast")
Chairman Koval of the Tal Shiar often claimed that there was a Federation spy in the Senate to deflect suspicion from himself, the Federation's operative in the Romulan government. (DS9: "Inter Arma Enim Silent Leges")
Several senators traveled to the front lines of the Dominion War, to represent the interests of the Senate. Senators such as Letant and Kimara Cretak represented the Romulan Empire on Deep Space 9. (DS9: "Tears of the Prophets", "Image in the Sand", "Shadows and Symbols", "Inter Arma Enim Silent Leges")
Following the Star Empire's placement of plasma torpedo launchers to protect its hospital on Derna, the Federation sent a formal protest to the Senate. The Senate retorted with a protest of its own. (DS9: "Shadows and Symbols")
In 2379, all but one member of the Romulan Senate were assassinated by Shinzon of Remus, who gained support from the Romulan military and declared himself Praetor. Following that incident, Captain Jean-Luc Picard expressed his gratitude to Shinzon for allowing him to be the first Federation officer to set foot of the floor of the Romulan Senate. The Senate was reconvened after Shinzon's death and the defeat of his Reman insurrection. (Star Trek Nemesis)