Cooperative learning is an evidence-based teaching strategy that has positive effects on students' academic and social learning, but teachers use the strategy in class infrequently. Teacher collaboration has the potential to support teachers in the transformation of knowledge into the implementation of new teaching methods. In an experimental pre-test-post-test design (N = 53), an in-service teacher training with teacher collaboration for the implementation of cooperative learning in class was compared to a training without teacher collaboration. At both points of measurement, teachers' knowledge about cooperative learning was tested and they reported on their self-efficacy beliefs for the implementation of cooperative learning and their use of cooperative learning in class, including the frequency, quality, and challenges of cooperative learning implementation. In both conditions, teachers' knowledge and self-efficacy beliefs, and the quality of their use of cooperative learning increased strongly, whereas perceived challenges decreased. A significant interaction effect occurred only for the frequency of teachers' use of cooperative learning in class. These results are discussed against the background of the efficacy of collaboration in in-service teacher training.