Betway's logo will feature prominently on the training kit for United's men's and women's teams, as well as around Old Trafford.
United went without a training kit sponsor last season after the expiry of their agreement with Tezos in June 2025.
Their return to the Champions League this season has allowed them to secure a more lucrative deal, reportedly the biggest in the world for a club's training kit.
The deal comes into effect at the same time as new Premier League regulations that prevent gambling companies from appearing on the front of matchday shirts from next season.
Announcing the Betway partnership, United's chief business officer Marc Armstrong said: "We are delighted to welcome Betway as official principal partner.
"The scale of our exciting new partnership reflects our growth strategy, the enduring global strength and appeal of Manchester United, and our ability to attract leading brands that want to engage with our vast worldwide fanbase."