This paper closes a gap in joint acceptance accounts of the mental life of groups by presenting a theory of group self-knowledge in the joint acceptance framework. I start out by presenting desiderata for a theory of group self-knowledge. Any such theory has to explain the linguistic practice of group avowals, and how self-knowledge can play a role in practical and moral considerations. I develop an account of group self-knowledge in the joint acceptance framework that can explain these desiderata. I (...) argue that a group has self-knowledge of its own attitude A if and only if (i) the operative members jointly accept that the group has attitude A; and (ii) that joint acceptance is non-deviantly caused by the operative group members’ belief that the members jointly accepted attitude A; and (iii) the group has attitude A. Together (i) to (iii) give us conditions for a group having a justified, non-lucky, true belief about its own attitude. (shrink)