East Fife will be playing Second Division football next season after David Muir's goal proved enough to inflict a first defeat in 12 matches on Peterhead.
Muir struck three minutes after the break in their second leg to register the only goal of the play-off at Balmoor after Wednesday's first leg finished goalless.
It was only the Fifers' third win in their last 21 games but it was enough to ensure Peterhead, who had won their previous 10 successive matches prior to the midweek stalemate, will find themselves in the Third Division once again next term having finished runners-up to champions Rangers.
Both sides showed their intentions early on, Rory McAllister firing off target for the hosts from distance before Paul Willis volleyed over for the Fifers.
Collin Samuel and McAllister, again, tried their luck before Peterhead striker Andrew Rodgers forced Calum Antell into a block in the 20th minute.
Opposite number Graeme Smith did well to tip Paul McManus' drilled effort around a post while Antell did likewise to keep out a free-kick from the lively McAllister at the other end as the first half drew to a goalless close.
The tie was finally blown wide open just three minutes after the break as Craig Johnstone's inswinging corner was converted right-footed from close range by Muir, whose third goal of the season followed his effort in East Fife's play-off semi-final victory over Berwick.
Smith again denied McAllister with another fine save before Peterhead's chances dried up, with the hosts racking up more bookings than goalscoring opportunities as the clash entered the closing stages.
McAllister, Peterhead's most likely source of an equaliser, twice fired off target in the final six minutes, with Willis denied a second for the Fifers by Smith.
Ross Smith then saw his injury-time header blocked by Scott Durie as Peterhead suffered their first defeat since the start of March and East Fife remained in the Second Division.