It was a disappointing return to Fratton Park for Steve Cotterill who watched his side concede three goals in spite of dominating for long periods.
Forest dominated much of the first half, looking the more confident and creating the majority of the chances, but it was the home side who took the lead in the 45th minute.
Forest had several chances to get back on top in the second half but again it was Portsmouth who found the back of the net to double their lead and add a third six minutes from time to put the game out of Forest's reach.
There was just one change to the side that started against Reading on Tuesday, with Jonathan Greening replacing Radi Majewski in midfield.
Portsmouth started brightly, winning two early corners, but Forest quickly took control of the play, looking the more dangerous side.Robbie Findley
showed his pace to get on the end of a flick on but his shot, under a challenge from Ricardo Rocha, went wide.
And Findley had another chance when he picked up a loose ball in the Portsmouth defence, but his effort from 25 yards went wide.
Forest continued to threaten and missed a great opportunity to take the lead when Chris Gunter
made a great run and crossed from the right to an unmarked Lewis McGugan
who shot from the penalty spot, but he couldn't control the ball and Stephen Henderson smothered it.
Forest pushed forward again with Marcus Tudgay and McGugan combining well, but when McGugan fired in the cross it went right through the Portsmouth penalty area.
It was 25 minutes before Portsmouth threatened the Forest defence when Erik Huseklepp made a great run down the left before being stopped by Luke Chambers.
Dave Kitson also created a chance but his shot took a deflection off Guy Moussi and went wide of the post for a corner.
Forest came close when McGugan fired in a free kick from long-range and Henderson did well to tip the ball over the crossbar for a corner.
The corner wasn't properly cleared by Portsmouth and Moussi chipped the ball back in, but straight at the Portsmouth keeper.
Portsmouth threatened late in the half when Abdul Razak got the ball through the Forest defence towards Luke Varney and Lee Camp was quick to get off his line but was not able to hold the ball and Wes Morgan did well to put it out for a corner.
Portsmouth took the lead just before the break following a free kick which was drilled towards goal from 20 yards by Hayden Mullins. It crashed off the corner of the crossbar and post and came back out to the left where Huseklepp found himself in plenty of space and curled the ball into the top corner of the net.
Portsmouth almost doubled their lead in the opening minutes of the second half when Razak got away and fed the ball to Varney who squared it back to David Norris whose effort was well saved by Camp.
And they threatened again soon after when Kitson picked up the ball on the corner of the penalty area but, under pressure from Camp, sent the ball over the crossbar.
Forest managed to get forward to win a corner and McGugan's effort found Chambers but his header was intercepted by Mullins who put the ball out for a corner.
And great delivery again by McGugan threatened the Portsmouth goal when his free kick came in through a crowd of players only to deflect off a Forest player and loook up over the crossbar.
Forest came close to getting the equalizer after a good ball in by Marcus Tudgay and great work by Findley who turned and stabbed the ball towards the top corner where Henderson produced a superb save to reach high and put the ball round for a corner.
A good break by Portsmouth saw Huseklepp make a superb run past two Forest players before Greg Cunningham got in to make the clearance.
Gunter missed a chance to get one back when Cunningham's cross was only cleared as far as the edge of the box where it fell for Gunter who got underneath the ball but sent it high over the bar.
With 14 minutes to go Portsmouth doubled their lead from their first corner of the half. It was good delivery in to the near post where Kitson rose high and nodded in.
David McGoldrick became Forest's first substitution of the game when he came on to replace Lynch with nine minutes remaining.
But six minutes from time Portsmouth had put the game out of Forest's reach when the cross came in from the right and out the other side before coming back in to Huseklepp who turned the ball in.
And they came close to a fourth when Greg Halford missed a chance from four yards out when he headed the ball into the ground and it bounced up and over the bar.
Nottingham Forest: Camp, Chambers(C), Morgan, Lynch(McGoldrick 81min), Cunningham, Gunter, Moussi, McGugan(Reid 85min), Greening, Tudgay, Findley(Moloney 90+1min)
Subs not used: Smith, Majewski,
Bookings: Tudgay 3min, Lynch 46min
Portsmouth: Henderson, Rocha, Pearce, Mullins, Huseklepp(Benjani 89min), Varney, Razak(Ward 62min), Halford, Kitson, Norris(c), Ben-Haim
Subs not used: Ashdown, Riise, Kanu
Scorer: Huseklepp 45min, 85min Kitson 76min
Referee: Tony Bates