Tatafu, 27, who was born in America Samoa and moved to Japan for high school studies, made the last of his 12 Test appearances against France in November.
"I will seize this opportunity and do my best to show my growth by applying what I have learned at Suntory Sungoliath," Tatafu said in a club statement.
"Without thinking about the future for now, I will do my best to win the championship in this League One season," he added.
Sungoliath are third in the Japan Rugby League One standings after three rounds of the season with Tatafu expected to feature for his country at this year's Rugby World Cup starting on September 8th.
"His departure from the team will be a huge loss for the team, but I believe that this challenge will lead to his personal growth," Sungoliath's coach Naoya Okubo said.
"We are currently in the midst of the League One season, but when the season is over, the Rugby World Cup 2023 in France will finally be held.
"We hope that he will take on challenges and show us how much he has grown," he added.
