Birtvisi (Georgian: ბირთვისი) is a ruined medieval fortress in Kvemo Kartli, Georgia, nested within limestone cliffs in the Algeti river gorge.

[1] 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.

[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.