The present study introduced "agentic engagement" as a newly proposed student-initiated pathway to greater achievement and greater motivational support. Study 1 developed the brief, construct-congruent, and psychometrically strong Agentic Engagement Scale. Study 2 provided evidence for the scale's construct and predictive validity, as scores correlated with measures of agentic motivation and explained independent variance in course-specific achievement not otherwise attributable to students' behavioral, emotional, and cognitive engagement. Study 3 showed how agentically engaged students create motivationally supportive learning environments for themselves. Measures of agentic engagement and teacher-provided autonomy support were collected from 302 middle-school students in a 3-wave longitudinal research design. Multilevel structural equation modeling showed that (a) initial levels of students' agentic engagement predicted longitudinal changes in midsemester perceived autonomy support and (b) early-semester changes in agentic engagement predicted longitudinal changes in late-semester autonomy support. Overall, these studies show how agentic engagement functions as a proactive, intentional, collaborative, and constructive student-initiated pathway to greater achievement (Study 2) and motivational support (Study 3).