Teacher design teams (TDTs) are increasingly used as a means for teacher professional development. It has been posited that for teacher learning to occur, TDTs need support from team coaches. These coaches are either external experts or peer teachers that guide the team from within. The current literature is in debate on whether external or internal coaches are most effective in supporting TDTs. In this study, we, therefore, examine whether these coach types differ in how they fulfil their role. We additionally evaluate how coaching interacts with the team learning process and the TDT trajectory's outcomes. We used a mixed methods design in the context of a large-scale TDT trajectory in Flanders (Belgium). We administered questionnaires among 63 teachers of 18 TDTs, and conducted interviews with the coaches of 14 TDTs. Our results indicate that coaching activities correlate with the majority of team learning beliefs and behaviours (TLBB) examined, as well as with perceived team effectiveness and the quality of material developed. Whereas teachers in TDTs with an internal coach seem to evaluate the coaching activities and the TLBB more positively than teachers in TDTs with an external coach, the opposite holds for perceptions of the trajectory's outcomes.