Asen's Fortress (Bulgarian: Асенова крепост, romanized: Asenova krepost), identified by some researchers as Petrich (Петрич), is a medieval fortress in the Bulgarian Rhodope Mountains, 2 to 3 kilometres (1.2 to 1.9 mi) south of the town of Asenovgrad, on a high rocky ridge on the left bank of the Asenitsa River.

[1] The earliest archaeological findings date from the time of the Thracians, the area of the fortress being also inhabited during the Ancient Roman and Early Byzantine period.

Renier of Trit would be besieged in the fortress for eleven months with just 40 knights before being relieved by the army of Latin Emperor Henry of Flanders.

[2] It was considerably renovated in the 13th century (more precisely 1231) during the rule of Bulgarian tsar Ivan Asen II to serve as a border fortification against Latin raids, as evidenced by an eight-line wall inscription.

It is a two-storey cross-domed single-naved building with a wide narthex and a large rectangular tower, and features mural paintings from the 14th century.