Chris Riggott went straight into the starting line-up on his return
Chris Riggott and Tuncay headed home corners to preserve Middlesbrough's top-flight status for another season.
Riggott, recalled from a loan spell at Championship promotion chasers Stoke City, headed home Fabio Rochemback's corner on 40 minutes to settle nerves.
And three minutes into the second half, Tuncay repeated the feat, flicking home Stuart Downing's inswinging delivery.
Portsmouth offered little, though Milan Baros was wasteful with two glorious chances inside the first 20 minutes.
But that will be of little concern to Boro, who bounced back from a run of just one win in 10 league matches to secure the three points they needed.
Gareth Southgate's side knew that, regardless of other results, they could secure their Premier League status by beating a Portsmouth side which had picked up just one point from three games and perhaps had half an eye on the FA Cup final with Cardiff City.
Gareth Southgate was relieved after seeing his side achieve safety
But given their dismal recent run and increasingly precarious league status, the home side started understandably nervously.
Portsmouth could have taken immediate advantage, Arnold Mvuemba pulling down Sylvain Distin's crossfield pass superbly and playing in a low cross, but Baros' shot was easily blocked, Glen Johnson firing the rebound well wide.
The fans in a half-empty Riverside Stadium might have hoped the early chance would be indicative of an exciting half. If so they were to be disappointed.
With chances and quality at a premium, it was unfortunate for Portsmouth that their best openings fell to Baros, a striker extending his run without a goal since signing on loan from Lyon to 14 matches.
And he was guilty of a glaring miss on 20 minutes, firing hopelessly over the bar from 12 yards after Mvuemba picked him out with another rolled cross.
Boro, the second lowest scorers in front of their own fans in the league, fed on scraps in the opening half hour, long range efforts from Downing and Afonso Alves as close as they came to troubling Jamie Ashdown.
But as Portsmouth became increasingly wasteful with possession, Boro grew in confidence and after 10 minutes of domination took the lead in the 40th minute.
The goal was simplicity itself, Rochemback floating in a corner and Riggott powering past Sylvain Distin to head home past a rooted Ashdown.
Despite not having won by two clear goals since September, Boro doubled their lead eight minutes into the second half and again the goal was straight off the training ground.
Downing took over corner duties and from his inswinging near-post delivery, Tuncay ran across his marker to flick a header past Ashdown's right hand and into the opposite corner.
Harry Redknapp was unhappy with Pompey's defending
Portsmouth, in truth, were shocking. After their early opening for Baros they created nothing of note until Niko Kranjcar ran at David Wheater, turned him, but shot wide from 14 yards.
That was enough for Harry Redknapp, who hauled off the anonymous Pape Bouba Diop and misfiring Baros to be replaced by Sean Davis and David Nugent.
Not that it made any difference and Downing should have added a third on 76 minutes, but he shot straight at Ashdown from six yards having been played in by a delightful Rochemback pass.
Middlesbrough manager Gareth Southgate:
"We can look forward to next season now. The players will be delighted and I'm sure this experience will make them all the stronger for next season.
"It's a tough division. We have maintained our status in it for almost 10 years which is an achievement.
"Chris (Riggott) has had a good spell on loan. It's to his credit that he came in and did so well. Tuncay worked really hard for us. He was a bit over-eager in the first half, but his work rate has been outstanding."
Portsmouth manager Harry Redknapp: "We started the game well and had two or three opportunities, but couldn't stick them away.
"They've scored from a set piece and we've come in at half time and said what's the point of having markers if they don't mark? Then they've done the exact same thing in the second half.
"The players have got to play well if they want to play in the Cup final. It was disappointing today, very disappointing.
"We need to try to win against Fulham next week. We'll pick a strong team, we owe that to everybody."
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