Many outsiders won on 16 and 17 March, offsetting a customer-friendly set of results on 14 March when five favourites were victorious.
Flutter's UK & Ireland brands subsequently ended the week with a profit.
On 14 and 15 March, two gamblers won £100,000 and £200,000 as part of the Sky Bet-sponsored and free-to-play promotion ITV7.
Meanwhile, an Irish Paddy Power retail customer in Kildare won €75,000 ($80,600) on a 20p "Lucky 63" wager that produced seven wins.
Dom Crosthwaite, Flutter's Chief Trading Officer, said: “It was another memorable Cheltenham, and it was pleasing this year to see the Festival's mass market appeal mirror the recreational nature of our customer base.
“We delivered a great customer proposition for our flagship brands, with a series of recent product upgrades and marketing campaigns driving strong staking levels of well over £250m.
"This was all made possible by our talented colleagues in Flutter UK&I, who worked incredibly well to seamlessly manage over 37m online bets across the week.”
Recently, a contract was inked by PA Betting Services and Flutter to extend their multi-year partnership.
According to the agreement, PA Betting Services will provide racing data to Flutter's brands, including Paddy Power and FanDuel.
Gambling Insider was present throughout last week at the Cheltenham Festival, including Gold Cup day.