Renier of Trit was the first Frankish duke of Philippopolis (modern Plovdiv, Bulgaria) from 1204 to 1205. He was a knight from Trith-Saint Léger, Hainaut, on the Fourth Crusade.
Renier was granted Philippopolis and the territory as far as the river Maritsa by the Emperor Baldwin I following the October 1204 partition of the conquered and yet to be conquered lands of the Byzantine Empire. Renier's land lay within the modern realm of Bulgaria, in territory claimed by Byzantium and subsequently the Franks.
Renier's first campaigns that fall and winter to take possession of his imperial fief were successful, but the next year Bulgarian tsar Kaloyan swooped down and took Adrianople and threatened Philippopolis. Renier, with only a small force remaining at his command, holed up in the castle of Stenimaka. It was during an effort to relieve Adrianople that the emperor Baldwin was captured. In the summer, the Paulicians of Philippopolis tried to surrender the city to Kaloyan, but Renier sallied from his fortress and razed their quarter of the city, leaving the rest to the brave defence of the united Latin and Greek populations. Nevertheless, the city was taken and the Greek quarter burned. Later that same year, the imperial regent Henry of Flanders marched into Bulgaria and relieved Stenimaka and Renier.