The study investigates one mathematics teacher’s implementation of formative assessment and its effects on students’ self-regulated learning (SRL). A questionnaire administered before and after the eight-month long intervention shows a significant effect, compared to two control classes, on students’ motivational beliefs involved in SRL. Qualitative data shows a notable enhancement of the students’ SRL behavior in the classroom. Analysis of the teacher’s implemented formative assessment shows a practice integrating several aspects of formative assessment, and provides empirical evidence of what formative assessment with large effects on students’ SRL may look like and how it fits with models of SRL development.