Tsuboi changed teams once again in 2021, joining TGR Team au TOM's alongside Yuhi Sekiguchi. Tsuboi finished second in the opening round at Okayama International Circuit, battling Kenta Yamashita for the lead for several laps. Heading into the final round at Fuji Speedway, Tsuboi and Sekiguchi trailed championship leader Naoki Yamamoto by 16 points.