To evaluate effectiveness of a standardized patient actor (SP) training intervention to improve quality of preexposure prophylaxis (PrEP) services for adolescent girls and young women (AGYW) in Kenya. Cluster randomized trial and mystery shopper evaluation. Twelve of 24 maternal child health and family planning facilities were randomized to SP training. Providers at intervention facilities participated in 2-day training in adolescent health, PrEP guidelines, values clarification, and communication skills, followed by role-playing and de-briefing with trained actors. Control facilities received standard national training. The primary outcome was quality of care, assessed by unannounced SPs (USPs) or "mystery shoppers" blinded to intervention arm. Quality was measured in two domains: guideline adherence and communication skills. Intent to treat analysis compared postintervention quality scores by randomization arm, clustering on facility, and adjusting for baseline scores and USP. Overall, 232 providers consented to USP visits, and 94 providers completed the training. Following training, USPs posed as AGYW seeking PrEP in 142 encounters (5-6 encounters per site). The mean quality score was 73.6% at intervention sites and 58.4% at control sites [adjusted mean difference = 15.3, 95% confidence interval (CI): 9.4-21.1, P < 0.001]. Mean guideline adherence scores were 57.2% at intervention sites and 36.2% at control sites (adjusted mean difference = 21.0, 95% CI: 12.5-29.4, P < 0.001). Mean communication scores were 90.0% at intervention sites and 80.5% at control sites (adjusted mean difference = 9.5, 95% CI: 5.5-13.6, P < 0.001). SP training significantly improved quality of PrEP care for AGYW in Kenya. Incorporating SP training and unannounced SP evaluation could improve PrEP uptake among AGYW.