A Boro statement said Mousinho rejected the offer of new terms at Adams Park in order to join Graham Westley's side.
Mousinho began his career at Brentford in 2005, playing 69 games, before switching to Wycombe two years ago.
The holding midfielder becomes Westley's fifth summer recruit, after wingers Peter Winn and Robbie Sinclair and defenders Luke Foster and Darius Charles joined the club.
Source: BBC Sport
Source: BBC Sport