WADA's director general Olivier Niggli confirmed at a news conference on Wednesday that the agency had filed an appeal at the Court of Arbitration for Sport (CAS).
"There was a decision taken by the swimming federation (FINA) and all I can confirm is that we've appealed that decision to CAS," he said.
Britain's Sunday Times newspaper reported in January on an alleged dispute between doping testers and the swimmer last year which the newspaper said had resulted in damage to a blood sample.
China's swimming association (CSA) rejected the allegations, saying an investigation by FINA had found the swimmer committed no anti-doping rule violation.
The CSA said Sun had rejected an out-of-competition doping test at his home in China on Sept. 4 last year over what the athlete said was the testing officials' lack of proof of identification.
Sun, a triple Olympic gold medallist and world record holder, was banned in 2014 for three months after testing positive for the banned stimulant trimetazidine during the Chinese national championships.
His lawyer Zhang Qihuai has threatened the Sunday Times with legal action, saying in a statement to the Chinese news agency Xinhua that the newspaper reported with "a malign intention" which "severely damaged Sun Yang's reputation and violated his privacy."
(Writing by Brian Homewood; Editing by Hugh Lawson)
