The move continues Chris Wilder's January reshaping of his squad with three new loan signings having made their debut against Southend United at the weekend.
Langmead departs after making over 100 Cobblers appearances, a figure which would have been significantly more had it not been for a serious knee injury suffered in March 2013.
Update: Ricky Ravenhill has also had his contract terminated while Alex Nicholls' deal has expired and he has sealed a permanent move to Exeter City.
Nicholls is eligible to play against the Cobblers this weekend.