The 34-year-old, who has won 45 caps for his country and played in the Premier League for Blackburn and Birmingham, will go straight into the squad for their FA Cup clash with Bromley at the Highbury Stadium.
"Barry is fantastic coup of the football club and a massive signing," chairman Andy Pilley said. "His footballing credentials are second to none and this is a huge, huge day for Fleetwood Town.
"Discussions have been held this week and Barry has really bought into the Highbury project, so much so he has taken a pay cut to drop into League Two.
"We are a hugely successful and progressive club and signing someone who has achieved so much in the game speaks volumes for our footballing aspirations."
Ferguson's first-team appearances have been limited this season under Ian Holloway.
Having played 45 times last season, the midfielder has made just seven appearances for the Seasiders so far in 2012/13.