The Pontifical Academy of Sciences invited Huang Jiefu, the man in charge of overhauling China's transplant system, to the two-day conference in the tiny city state.
Wendy Rogers, a medical ethics expert at Macquarie University in Australia and the chair of an advisory committee on tackling organ theft in China, slammed Huang's presence as "shocking".
Huang told journalists at the summit that the controversy was "ridiculous" and the use of organs from executed prisoners in China "is not allowed under any circumstances".
He admitted though that organ transplants from prisoners may still be taking place.
"There is zero tolerance. However, China is a big country with a 1.3 billion population so I am sure, definitely, there is some violation of the law," he said.
Both the European Parliament and US Congress have recently condemned organ harvesting in China amid widespread concerns tissue is sourced from executed prisoners of conscience.
Victims reportedly include not only death-row prisoners but also religious and ethnic minorities such as Uighurs, Tibetans, Falun Gong spiritual practitioners and "underground" Christians.