Daiya Seto is the reigning world champion in both the 200 and 400 meter medley.
He is one of Japan's big hopes for the home Olympics in Tokyo next year.
But the preparations will not be optimal as he has now received his punishment for the extramarital affair which he admitted at the end of September.
His actions violate the rules of the Japanese federation and he will now not be allowed to compete again in 2020.
- How can I apologize?
I have asked myself if it is possible, but I think my excuse will be to continue swimming and to regain my family's trust.
"My family has been deeply hurt by my irresponsible behavior," Seto told Hochi Sports, according to Reuters.
Seto, who took bronze at the Olympics in Rio de Janeiro in 2016, has already announced that he is resigning from his role as captain of the Japanese national swimming team at the Olympics.
A large airline has also broken its sponsorship contract with the swimmer.