This research has a dual purpose: to translate into Spanish and validate a classroom engagement measure and, over a semester, to analyse the effect of students' perception of autonomy support on the need for autonomy and the effect of autonomy, in turn, on four types of engagement. Data were collected at three time points from 448 undergraduate students via a longitudinal design. The results revealed adequate psychometric properties for the engagement scale, and the hypothesised effects were supported. Autonomy support was a significant predictor of the need for autonomy, which, in turn, predicted changes in four types of classroom engagement. Emotional engagement displayed the strongest relationship with need for autonomy. Moreover, need for autonomy mediated the relationship between perceived autonomy support and each indicator of student engagement. The findings are interpreted as supporting self-determination theory's motivation mediation model and could be considered in future intervention programmes to improve the teaching-learning process in education.