And the Bournemouth stopper goes straight into Paul Cook’s team for the play-off first leg against Plymouth tonight.
The Blues had been left with no fit keepers after Brian Murphy and Paul Jones were declared injured.
Current first choice Murphy picked up a problem in last weekend’s defeat to Northampton.
In addition, Jones is still not ready to play following a thigh tear, despite being named on the bench against the Cobblers.
Academy keeper Alex Bass remains on the sidelines with a knee injury, while Ryan Fulton is still receiving treatment with parent club Liverpool.
The Football League gave the go-ahead to recruit a keeper on an emergency loan deal on Monday.
However, manager Paul Cook had gone to lengths to keep the fresh injury developments quiet ahead of tonight’s visit of the Pilgrims.
Now Pompey have recruited Allsop, who made 18 appearances on loan at Wycombe during the second half of the campaign.
The 23-year-old, who trained with his new team-mates yesterday, was part of the Chairboys side which lost 2-1 at Fratton Park last month.