There is a critical need for evidence-based and manualized interventions targeting water competency including swim and water safety skills tailored to meet the needs of children on the autism spectrum, a group that is at a high risk of drowning. This study examined the efficacy of AquOTic-a 10-week occupational therapy-based aquatic intervention to improve water competency among children on the autism spectrum. A total of 37 children on the autism spectrum (ages 5-9 years) were randomized to a waitlist control group (n = 24) or AquOTic intervention group (n = 37; 28 males). Blinded assessors administered the standardized Water Orientation Test-Alyn (WOTA) 1 and 2 and a Swim Skills Checklist to all participants pre- and post-AquOTic/control. Repeated measures mixed effects models were used to examine intervention effects. Average WOTA 1 scores increased significantly after participants received AquOTic (Δ = 5.7; 95% CI: 3.7-7.8; p < 0.001), and average WOTA 2 scores increased significantly after participants received AquOTic (Δ = 9.0; 95% CI: 5.7-12.3; p < 0.001). Average swim skills increased significantly after participants received AquOTic (Δ = 7.6; 95% CI: 5.3, 10.0; p < 0.001). Our results highlight the efficacy of AquOTic to improve water competency among children on the autism spectrum. Further research is needed to examine long-term effects, dosage requirements to achieve water competency, and the impact of aquatic therapy on other health outcomes. clinicaltrials.gov, NCT05524753.