English: Birtvisi Natural Monument is a ruined medieval fortress in Kvemo Kartli, Georgia, nested within limestone cliffs in the Algeti river gorge. It is now within the boundaries of the Tetri-Tsqaro municipality, adjacent to the Algeti National Park, south-west of the nation's capital Tbilisi.

Birtvisi is essentially a natural rocky fortress of 1 km², secured by walls and towers, the most prominent of which – known as Sheupovari ("Obstinate") – tops the tallest rock in the area. Various accessory structures, an aqueduct included, have also survived.[2]

In written sources, Birtvisi is first mentioned as a possession of the Arab amir of Tiflis of which he was divested by the Georgian nobles Liparit, Duke of Kldekari and Ivane Abazasdze in 1038.[3] In medieval Georgia, Birtvisi entertained a reputation of an impregnable stronghold whose master could control the entire strategic Algeti gorge. The Turco-Mongol amir Timur notably reduced the fortress during one of his invasions of Georgia in 1403.[4] After the partition of the Kingdom of Georgia later in the 15th century, Birtvisi was within the borders of the Kingdom of Kartli and in possession of the princes Baratashvili.[2]