BackgroundStudent engagement and subjective task value belief are critical psychological constructs driving the second language (L2) learning process. L2 research has established the positive effect of subjective task value belief on student engagement in the L2 classroom, while the reverse effect has received some theoretical and empirical support in the broader field of educational psychology. However, theoretically grounded empirical work on testing the reciprocal relationship between these two constructs remains absent in L2 research. AimThis study sought to examine the longitudinal reciprocal relationship between L2 students’ classroom engagement and their subjective task value belief – the relationship that is neither sufficiently theorized nor empirically tested in the L2 learning context. SampleData were collected across three time points over an academic semester from 920 undergraduate students learning English as a foreign language in Vietnam. MethodRandom intercept cross lagged panel modelling was conducted to examine the carry-over (or autoregressive) effects of L2 students' subjective task value belief (or their classroom engagement) at one time on itself at a subsequent time, as well as the concurrent effects and spill-over (or cross-lagged) effects of L2 students’ subjective task value belief on their classroom engagement and vice versa. ResultL2 students’ classroom engagement and their subjective task value belief not only co-varied within individuals concurrently, but variation in one construct led to subsequent variation in another over the semester. ConclusionFindings confirm the dynamic, situation- and time-specific patterns of relationship between the two constructs in line with the expectancy value theory (Eccles & Wigfield, 2020a) and the development-in-sociocultural context model of student engagement (Wang, Henry, & Degol, 2020).