A process chamber and a substrate processing apparatus including the same are disclosed. The process chamber includes a first housing and a second housing on the first housing. The first housing includes a first outer wall, a first partition wall facing the first outer wall, and a first side wall connecting the first outer wall and the first partition wall. The second housing includes a second outer wall, a second partition wall between the second outer wall and the first partition wall, and a second side wall connecting the second outer wall and the second partition wall. Each of the first and second outer walls has a thickness greater than a thickness of the first partition wall and a thickness of the second partition wall.