This article focuses on the impact of a specific formative assessment procedure, negotiated assessment, on teacher professional learning. Negotiations between the assessor and the teacher as assessee seem to be especially promising for this teacher learning. However, there is no empirical evidence yet that has confirmed this. We explored teachers' opinions about the usefulness of the different elements in a negotiated assessment procedure for their professional learning and the learning benefits they reported as a result of being engaged in this procedure. Our findings show that teachers found the negotiated assessment procedure useful for their learning and reported different types of learning benefits in terms of change: change in their knowledge, beliefs and attitudes, change in their teaching practice and change in their students' learning outcomes. (Contains 5 tables and 1 note.)