The 22-year-old has linked up with the Pilgrims on a permanent deal – having helped Church reach the second round of this season's Emirates FA Cup, beating League One side Cheltenham Town in the process.
Sephton has scored twice in 19 appearances so far this season and is eligible to make his United debut against Kettering Town on Saturday.
He will wear the number 3 shirt at the Jakemans Community Stadium.
Elsewhere, on-loan full-backs Joe Starbuck and Harry Lewis have returned to Sheffield United and Scunthorpe United respectively.