The Kyodo news agency said that 39-year-old Terauchi, who is being treated in hospital, suffered initial symptoms of the virus on July 25 and had a fever measuring over 37 degrees celsius on July 27 and 28, writes Xinhua news agency.
He underwent a polymerase chain reaction, or PCR, test last Friday after finding out that an acquaintance he met after training in Osaka on July 24 tested positive for the virus, Kyodo said.
Terauchi qualified for the Tokyo Olympics along with teammate Sho Sakai after finishing seventh in the men's 3-meter synchronised competition at last year's world championships in Gwangju, South Korea.
Kyodo said he is so far the only member of the Japanese Olympic team known to have been infected with the coronavirus.
Japan has been suffering a second wave of infections since early July, with confirmed cases increased by 955 on Monday, including 258 cases in Tokyo.