The Blues won the game 3-1, with all the goals coming in the first half.
Joe Pigott handed the hosts the lead on 23 minutes in controversial circumstances.
But Pompey responded a minute later through Ronan Curtis, before two late first-half strikes from Lee Brown secured the win that keeps the Blues sixth in the table and in control of their own play-off destiny after Charlton could only draw at Accrington.
A message from the Editor, Mark Waldron
Get 25% off the cost of The News’ annual sports subscription,
You can subscribe here to get the latest news and transfer updates from Fratton Park - and to support our local team of expert Pompey writers.
Use code PROMORACE21 at the checkout.