France faced Hungary - and played a draw - in Budapest, where the Hungarians had a large following in the stands.
During the match, there must have been applause from the host country's supporters.
This is stated by the French newspaper L'Equipe.
It must have been heard by photographers at the stadium.
The sounds must have arisen mainly when the ball was at the feet of N'Golo Kantés or Paul Pogba.
An investigation has been launched, Uefa writes in a press release.
"Pursuant to Article 31 (4) of UEFA's disciplinary regulations, a UEFA ethics and disciplinary inspector has been appointed to conduct a disciplinary inquiry into any discriminatory incidents that occurred at Puskas Arena, Budapest, during the EC group stage matches between Hungary and Portugal on 15 June 2021 and between the national teams Hungary and France which was played on 19 June 2021 "
Homophobic banner
The Guardian has previously written that a homophobic banner was seen in the arena in the match between Hungary and Portugal on June 15.
That match is also being investigated, which appears in the press release.