Formative assessment has been suggested as a means of supporting studentmotivation. However, empirical studies have shown mixed effects of formativeassessment interventions on students’ motivation, making it necessary to understandthe mechanisms underlying these effects. We analyzed a formative classroompractice implemented by a 10th-grade first-language teacher during 7 months.Teacher logs, classroom observations and a teacher interview were used to collectdata for characterizing the formative assessment practice. Changes in students’satisfaction regarding the basic psychological needs of perceived autonomy,competence and relatedness, as well as changes in student motivation manifestingas engagement in learning activities and autonomous types of motivation, weremeasured by pre- and post-questionnaires in the intervention class and fourcomparison classes. Since the intraclass correlation values ICC(1) and ICC(2)were low, we treated the comparison classes as one group and t-tests were usedin the significance testing of the differences in changes in psychological needssatisfaction and motivation between the intervention class and the comparisonclasses. Path analysis was conducted to investigate whether a possible influenceof the intervention on autonomous motivation and behavioral engagementwould be mediated by basic psychological needs satisfaction. The analysis ofthe classroom practice in the intervention class identifies that both teacher andstudents were proactive agents in formative assessment processes. The analysisof the quantitative data shows that students’ psychological needs satisfactionincreased more in the intervention class than in the comparison classes, and thatthis needs satisfaction mediated an effect on students’ behavioral engagementand autonomous motivation.