It was an upsetting result for last season's Turkish champions.
The Black Eagles took the lead in the 12th minute at Recep Tayyip Erdogan Stadium in Istanbul when Can Bozdogan fired a classy left-footed strike outside the penalty area to draw first blood.
Kasimpasa forward Umut Bozok scored the equalizer in the 82nd minute, making a close range finish after Eren Elmali's low cross from left.
Besiktas has underperformed in their last five league matches, with four losses and a draw.
The Black Eagles last won when they beat archrivals Galatasaray 2-1 at Vodafone Park on Oct. 25.
Besiktas then lost to Atakas Hatayspor, Trabzonspor, Aytemiz Alanyaspor and GZT Giresunspor, respectively.
They are in the ninth position with 21 points in 15 league matches.
Kasimpasa have 11 points and are in the relegation zone.
