A collapsible seat assembly is mounted on a frame of a stroller, and includes a joint member, a backrest support member, a leg support member and a locking mechanism. Each of a pivot portion of the backrest support member and a pivot portion of the leg support member is pivotably mounted on the joint member. The locking mechanism is disposed between the pivot portions to lock or unlock the backrest and leg support members. The locking mechanism is configured to be actuated to move from a locking position to an unlocking position by the operation of the frame of the stroller from an unfolded state to a folded state. A stroller having the collapsible seat assembly is also disclosed.