Reports of such a group in the Curia have circulated for months and may have been a consideration in Pope Emeritus Benedict XVI's retirement, CNN reported.
The Curia is the Vatican's bureaucracy.
A Chilean website, Reflection and Liberation, which discusses liberation theology, first reported Francis' comments, though CNN said a Vatican spokesman refused to comment on the report.
"In the Curia," Francis said, "there are holy people. But there is also a stream of corruption," adding, "The 'gay lobby' is mentioned, and it is true, it is there. We need to see what we can do."
The pope's remarks came during a meeting Sunday with CLAR, the Latin American and Caribbean Confederation of Religious Men and Women. Members head Catholic communities of priests, sisters and monks.
Some Catholics said people should not read too much into the pope's remarks, CNN reported.
"We don't have any explanation of what 'gay lobby' means," Rocco Palmo said. Palmo is a Vatican watcher who runs Whispers in the Loggia, a website on Catholic news and church politics.