WASHINGTON—Donald Trump’s eldest son, emerging as a potential envoy for the president-elect, held private discussions with diplomats, businessmen and politicians in Paris last month that focused in part on finding a way to cooperate with Russia to end the war in Syria, according to people who took part in the meetings.

Thirty people, including Donald Trump Jr., attended the Oct. 11 event at the Ritz Paris, which was hosted by a French think tank. The founder of the think tank, Fabien Baussart, and his wife, Randa Kassis, have worked closely with Russia to try to end the conflict.

Ms. Kassis, who was born in Syria, is a leader of a Syrian group endorsed by the Kremlin. The group wants a political transition in Syria—but in cooperation with President Bashar al-Assad, Moscow’s close ally.

The disclosure of a meeting between the younger Mr. Trump and pro-Russia figures—even if not Russian government officials—poses new questions about contacts between the president-elect, his family and foreign powers. It is also likely to heighten focus on the elder Mr. Trump’s stated desire to cooperate with the Kremlin once in office.

In an interview, Ms. Kassis said she pressed the younger Mr. Trump during the meeting on the importance of cooperating with the Russians in the Middle East.