The 25-year-old was previously with the Pars in 2017/18 before going on to join Ross County and now returns on the back of a successful loan spell at Falkirk.
McManus, who began his career at Aberdeen and has also played for Fleetwood Town and Morton, told the club website: "I feel there is unfinished business for me here. I really enjoyed my year here and leaving wasn’t an option that was easily taken.”
Dunfermline boss Stevie Crawford has also snapped up forward Kevin O'Hara from Alloa Athletic - the 21-year-old former Falkirk player has agreed a three-year deal.
Winger Dom Thomas became the club's third new signing of the day - he was on loan from Kilmarnock in the second half of last season and has now signed a two-year contract.