In this study, we investigated how student engagement and disengagement change over the course of a semester in the L2 classroom. We modeled change at the inter- and intra-individual levels, using time-variant predictors to examine differences in student classroom engagement and disengagement trajectories. In addition to these temporal dynamics, we also examined what motivational antecedents are related to these changes in engagement and disengagement over time. We collected data from 686 students enrolled in general-purpose English courses at two publicly funded universities in mainland China at three waves in a 17-week semester, and tested a series of multi-level, mixed-effects growth models. Our analyses showed that students who reported higher initial classroom engagement or disengagement levels had lower growth rates than their counterparts as the semester proceeded. Students' classroom engagement in language learning dipped to its lowest point around the middle of the semester and peaked toward the end of the semester. Motivational antecedents were also strong predictors of student engagement and disengagement in the language classroom at both within- and between-person levels. We discuss the implications of these temporal dynamics of learner engagement in the language classroom.