Changes in motivation anticipate changes in engagement, but the present study tested the reciprocal relation that changes in students' classroom engagement lead to corresponding longitudinal changes in their classroom motivation. Achievement scores and multiple measures of students' course-specific motivation (psychological need satisfaction, self-efficacy, and mastery goals) and engagement (behavioral, emotional, cognitive, and agentic aspects) were collected from 313 (213 females, 100 males) Korean high school students using a 3-wave longitudinal research design. Two key findings emerged from a multilevel structural equation modeling analysis: (a) Students' initial classroom engagement predicted corresponding longitudinal changes in all 3 midsemester motivations, and (b) early semester changes in engagement predicted corresponding longitudinal changes in end-of-semester psychological need satisfaction and self-efficacy, but not mastery goals. Changes in engagement also predicted course achievement. These findings reveal the underappreciated benefits that high-quality classroom engagement contributes to the understanding, prediction, and potential facilitation of constructive changes in students' in-course motivation.