This study used a randomized controlled design to test the pathways in basic psychological needs theory, where social relationships characterized by autonomy support, involvement, and structure foster psychological need satisfaction and well-being. Participants were recruited from a physical-activity-based youth program. A new staff training was implemented to manipulate the use of each interpersonal characteristic by program staff (N = 24 observed) and perceptions of each interpersonal characteristic, psychological needs, hope, and self-worth in youth (N = 379 surveyed pre- and postprogram). Staff in the intervention condition used greater overall observed autonomy support, involvement, and structure. Condition assignment did not lead to differences in youth perceptions, but observed staff behaviors positively predicted youth perceptions of staff and perceptions of staff positively predicted change in well-being. Findings indicate that the training manipulated how staff engaged youth, and autonomy support, involvement, and structure are useful strategies to foster well-being in youth.