The reigning Olympic and world all-around champion became hooked on the smartphone game after touching down in Brazil to prepare for the defence of his title.
But to Uchimura's horror he was hit with a 500,000 yen ($5,000) bill after discovering that he did not have a flat rate for overseas data usage.
"500,000 yen..." the defending six-time world champion was quoted by Kyodo news agency as saying.
Team-mate Kenzo Shirai confided that Uchimura was shellshocked by the charges.
Uchimura, though, escaped the hefty bill after contacting his phone company to plead his case, eventually only having to pay 3,000 yen.
"I really lucked out," Uchimura said.