A ball chair has a seat, a backrest assembly and a ball cushion. The seat has a top face, a passage defined in the top surface of the seat, at least three legs formed on the seat and each leg and a supporting base formed on seat. The backrest assembly mounted on the seat and has a bracket, an angle adjusting device, a locking device and a fixed arm. The bracket has two supporting tabs defined in the bottom of the bracket to abut against the supporting base. The ball cushion is mounted inside the passage in the seat. The ball chair provides functions as adjusting backrest angle and the backrest may be extended for convenient use.