The former caretaker boss at Queens Park Rangers, 39, takes temporary charge of the Chairboys after boss Gary Waddock was relieved of his duties following Saturday's 1-0 loss to AFC Wimbledon.
Joint-chairman Don Woodward said: "I'd like to thank Gary for his efforts and wish him every success for the future, but we felt it was time for a change.
"Gareth's a leader and a winner. He's an inspirational figure who has the full respect of everyone connected with the club and it was an easy decision to make.
"Wycombe Wanderers is a fantastic football club with a great set-up, squad and fan base, so there's no surprise to us that we've received a number of applications from high-calibre managers."