Niang, who enjoyed a loan spell in the Premier League with Watford in 2016-17, was caught by stewards entering the Marseille Pont-de-Vivaux racecourse to watch his horse "Swetty Beauty."
The French government allowed horse racing to restart behind closed doors on May 11 amid the coronavirus pandemic but Niang was among four people who attended the race track the following day.
A report shown in Equidia said the president of France Galop's south-eastern regional committee Patrick Dreux led Niang and three others into the car park using an access code before entering the deserted panoramic restaurant.
All five men were fined but said they would not have accessed the racecourse if there had been sufficient security staff or a sign making them aware of the national ban.
Dreux, who is a member of France's horse racing governing body, is reported to have told a steward: "You should not think that you're at home here, you're not the only person who knows the access code."
Niang is not the only Ligue 1 player to find himself in trouble during the lockdown period in France. Angers attacking midfielder Farid El Melali apologised after he was arrested and charged with public masturbation earlier this month.
The Professional Football League (LFP) in France cancelled the 2019-20 season last month, declaring Paris Saint-Germain as champions while Niang's Rennes securing a place in the Champions League qualification rounds.