In this study, we investigated two core understandings related to the nature of second language (L2) student engagement from a self-determination theory framework: (1) that there is a dual process of development that follows from the initial conditions of the L2 classroom environment (specifically teacher motivational practice: autonomy-supportive vs. controlling), which leads to two distinct motivational experiences (need satisfaction vs. need frustration), which in turn results in qualitatively different types of student classroom functioning (engagement vs. disengagement); and (2) that there are reciprocal effects between the L2 classroom environment and student classroom functioning. We collected data from 1,742 students enrolled in general-purpose postsecondary English courses in mainland China at three waves in a 17-week semester, and tested a longitudinal dual-process, reciprocal-effects model. Our analyses showed that student perceptions of teacher motivational practice at the beginning of the semester predicted psychological need satisfaction at later time points. Psychological need satisfaction in turn predicted later classroom engagement. Student engagement also had feedback effects and predicted subsequent perceptions of teacher motivational practice. We discuss the implications of these results for L2 learning and teaching and consider ways that future research might build on our design and extend these findings.