Noel Hunt fired Reading to a seventh straight win to lift them to within a point of second-placed West Ham.
Hunt headed in from Jason Roberts's cross midway through the opening period as the Royals maintained their push for automatic promotion.
Youngster Scott Allan was financially-troubled Portsmouth's main threat.
Michael Appleton's team dropped to the bottom of the Championship table after extending their run without a win to seven matches.
Reading controlled the game from the start but Alex Pearce and Kaspars Gorkss missed with early headers.
Brian McDermott's side took the lead when Ian Harte fed Roberts, whose cross from the right was powerfully headed past Stephen Henderson by Hunt.
The hosts should have doubled their advantage just before the break when Hal Robson-Kanu headed straight at Henderson from Jobi McAnuff's dangerous cross.
Allan was denied by a fine block after a clever run just after the hour before Luke Varney saw his follow-up charged down.
Allan failed to test Reading goalkeeper Adam Federici from 40 yards and 30 yards before Hunt hit the side-netting following another fine ball from McAnuff.
Reading manager Brian McDermott told BBC Radio Berkshire: "I thought we were excellent in the first half and scored a really good goal, but I don't think we had enough control of the game in the second 45 minutes.
"We've kept another clean sheet and the most important thing to do at this stage of the season is to win.
"We closed the game out well and it was always going to be tough against Portsmouth.
"There is still plenty of potential in this side and another gear that we can go up to. The players don't get the credit they deserve but hopefully they will at the end of the season.
"Our fans should be really proud of the group of players and there is a buzz around the place. That is what you want."
Portsmouth Michael Appleton: "I'm delighted with the way they played. The one thing lacking at the minute is that little bit of belief in the final third. That comes when confidence isn't as high as it should be.
"At the minute it's a difficult scenario for us because of where we are but I asked the players to give me everything and they applied themselves really well.
"I'm convinced and I know the players are still convinced that we've still got a good chance of staying up. There are 12 games left and we know how many we have to win to get out of this mess."