Mieczysław Kościelniak (born 29 January 1912 in Kalisz, died 5 March 1993 in Słupsk) was a Polish painter, graphic designer, and draftsman, brother of Władysław and Tadeusz Kościelniak.

Already a prominent artist, he was arrested in 1941 and sent to the Auschwitz concentration camp. His camp number was 15261. In the camp he drew about 300 paintings depicting the everyday life of prisoners. His work is currently displayed in the Auschwitz-Birkenau State Museum. During his imprisonment in Auschwitz, he met fellow prisoner Bronislaw Czech, a three-time Polish Olympian, and helped refine Czech's artistic talents. He also befriended Maximilian Kolbe, a Catholic priest who died in Auschwitz and was canonized by the Catholic Church in 1982.

Kościelniak became a member of the Association of European Culture (SEC), and was awarded the National Education Committee Medal, the Gold Cross of Merit, and the Medal in the Service of Polish Education.

After the war, Kościelniak moved to Warsaw, then to Ustka in 1979 and finally, in 1989, to Słupsk. He died in 1993 and is buried at the Ustka cemetery. One of the streets of the city now bears his name.