After his record-setting 2018 Japanese F3 campaign, Tsuboi made the step up to the Super Formula Championship in 2019, joining JMS P.mu/Cerumo-INGING. He recorded a best finish of second in a wet race at Fuji Speedway. Tsuboi finished 12th in the championship, and was third in the Rookie of the Year cup behind Álex Palou.