Cystothalamia is an extinct genus of sea sponges in the family Guadalupiidae that existed during the Permian and Triassic in what is now Austria, Hungary, Slovenia, Tajikistan, Croatia, Iran, Italy, Russia, Thailand, Tunisia, the United States (New Mexico and Texas), and Venezuela. It was described by G.H. Girty in 1909, and the type species is Cystothalamia nodulifera.