BACKGROUND: Ammonium persulfate (APS), an oxidizing agent used in hair products, manufacturing, and pool/spa water, can cause skin reactions, including allergic contact dermatitis. OBJECTIVE: To characterize positive patch test reactions to APS (2.5% petrolatum). METHODS: Retrospective analysis of patients tested to the North American Contact Dermatitis Group screening series from 2015 to 2018. RESULTS: Of 10,526 patients, 193 (1.8%) had positive patch test reactions to APS. Compared with APS-negative patients, APS-positive patients were significantly more likely to be male (43.2% vs 28.0%; P < .0001); have primary hand dermatitis (30.2% vs 22.0%; P = .0064), scattered generalized dermatitis (25.5% vs 17.9%; P = .0064), or trunk dermatitis (8.9% vs 4.9%; P = .0123); and have dermatitis that is occupationally related (22.2% vs 10.9%; P < .0001). More than half of the APS-positive reactions were currently relevant (57.0%); 19 (9.8%) were related to occupation, especially hairdressers (68.4%). Swimming pools/spas (23.3%) and hair care products (19.2%) were the most common sources of APS. LIMITATIONS: Immediate reactions and follow-up testing were not captured. CONCLUSION: The proportion of patients positive to APS was 1.8%. APS positivity was significantly associated with male sex and hand dermatitis. Swimming pool/spa chemicals were important sources of APS exposure.