A bearing seat for a ball joint which simplifies the process of joining a housing and a bearing seat and makes it easy to guarantee the stiffness necessary for a ball joint as a whole by making it easy to fill the interior of an undercut groove with a softened or molten material and a ball joint having this bearing seat for a ball joint are provided. The ball joint 100 has a bearing seat in the form of a ball seat 130 which receives a ball portion 113 of a ball stud 110 inside a cast housing 120 so that the ball portion 113 can slidably rotate. The ball seat 130 has an undercut groove 140 in its outer peripheral surface which contacts the inner peripheral surface of the housing 120. The undercut groove 140 is formed so that the inner width W2 of its bottom portion 142 is greater than the opening width W1 of the opening 141 and so that its depth D is shallower than the opening width W1 of the opening 141.