In a TikTok video clip, he said he was surprised by the positive test because he had already received two Covid-19 vaccine jabs.
He added that he had rarely gone outdoors and did not stay close to his family.
"Recently, I went to Pakkred Police Station in Nonthaburi province in a lottery fraud case and took pictures with villagers in the area," he said.
He apologised to people for causing trouble, asking them to take Covid-19 tests as soon as possible.
He added that he was waiting for an ambulance to take him to the hospital for medical treatment.
Somrak won the gold medal in the featherweight category at the 1996 Atlanta Olympics.