Youth-led Participatory Action Research (YPAR)—in which young people conduct research aimed at improving problems in their schools and communities—is increasing in public health, youth development, and education. We report on the development and psychometric testing of the YPAR Process Template (YPT)—to assess the quality of key YPAR processes in a systematic, flexible manner. Pairs of raters independently coded 40 live and videotaped observations. All scales achieved good to excellent interrater reliability with the exception of the power sharing over major decisions scale, which had interrater reliability in the acceptable range. This instrument can be useful for a range of settings practicing YPAR and similar youth empowerment programs. We further report findings generated by YPT assessments that demonstrate that power sharing was a robust predictor of observed youth engagement in the YPAR classes after controlling for the classes’ baseline level of engagement.