The 20-year-old made his Black Cats debut on the final day of last season and has previously enjoyed loan spells at AFC Wimbledon and Woking.
Knott made a big impact at both of those clubs, scoring eight times in 20 appearances for Wokinglast season and scooping AFC Wimbledon’s Young Player of the Year for 2011/12, despite only joining midway through the campaign.
Gareth Ainsworth said of the England U20 international: “Billy’s a forward-thinking midfielder. He can play out on the left, in the No.10 role and in central-midfield.
“He’s left-footed, got great quality on the ball and is tenacious in the tackle. He’s been making big noises up in Sunderland in their U21 squad.
“For us to be able to attract a player away from a Premier League club is brilliant.
“The fact that he wants to come here and play games for Wycombe says volumes about him too.”
He added: “He’s someone we’ve been watching for a while and we admire him as a player.
“Stuart Lewis is suspended and with Matt Bloomfield still one or two weeks away, it’s great to add to be able to add Billy to our midfield options.”
Ainsworth concluded: “I’d like to thank Kevin Ball, Paolo Di Canio and Sunderland for allowing Billy to come here and I’m looking forward to welcoming him into the group.”