De Roon moved to Teesside in June 2016 for an initial £8.8m fee and played 33 Premier League games last term.
The Netherlands international, 26, scored five goals in 36 league and cup games during his spell at Boro.
Meanwhile, manager Garry Monk confirmed the Championship club's interest in signing Chelsea midfielder Lewis Baker.
"He's a target and one of many that we've had throughout the summer months," said Monk.
Find all the latest football transfers on our dedicated page.

Inspired by fond memories of his childhood caravanning holidays in Southwold, Suffolk, photographer Gareth Iwan Jones turned his attention to this subject, the very British world of Caravanning.
"I've always thought of caravanning as a wonderfully eccentric, colourful world of quirky characters and interesting regalia that would make for a great photography project," says Iwan Jones. "It didn't disappoint."
Together with his wife, Iwan Jones stayed at more than 30 sites across the UK, from beauty spots to those not so picturesque spots near motorways, covering more than 2,000 miles.
"There is a unique union of canopy, awning and torrential rain that one can only experience in caravan and camping parks," he says.
"For me, the most endearing caravanning moments came with the worst weather - this is when your neighbours and fellow campers brave the elements to help you find your guy ropes and hammer in your pegs."
Iwan Jones found there was a daily routine to the caravanning life and would wander the site looking for good pictures as well as arranging to meet the campers later to take their portrait.
"Everyone was so enthusiastic and eager to be in the pictures, not a single person I approached declined to be photographed," he says.
"In my experience of photographing people in their environments, this is pretty much unheard of and a real reflection of the culture of caravanning - very open and friendly.
"The portraits were all shot with the same camera and lens combination, a Pentax 645 and 55mm lens and the same artificial lighting."
"I was deliberately looking to bring continuity to the posed portraits, as I knew the documentary side would be quite loose in its appearance.
"I'm always interested in finding the humour in my images, and the world of caravanning made for a great fit.
"From a man who takes his budgies caravanning to the varied attempts to bring a bit of glamour to one's caravan, there was plenty for me to photograph."
Caravan, A Great British Love Story by Gareth Iwan Jones will be published in early June by Frances Lincoln. You can see more of Gareth Iwan Jones's work on his website.
All photographs © Gareth Iwan Jones/Institute

The blast happened in a cafe in the town of Velez-Malaga at about 17:00 GMT during a local festival.
Pictures from the scene show severe damage to the La Bohemia cafe and debris strewn across a street.
Local newspaper La Opinion de Malaga said that the chef had raised the alarm moments before the blast.
The newspaper also reported (in Spanish) that police cars helped ferry the injured to hospital because the local ambulance service was overwhelmed with casualties.
The cause of the blast "could have been a gas leak," the town's mayor, Antonio Moreno Ferrer, told local reporters.

Foden, 26, scored his third try in two matches as Saints beat Quins 24-3 in the Aviva Premiership.
But he told BBC Radio Northampton: "If he continues to perform, I'm going to have to push myself to the limit.
"Hopefully it'll bring out the best in both of us and put England rugby in a good place."
Interim England coach Stuart Lancaster will name his squad for the Six Nations on Wednesday, with Brown expected to return for the first time since the tour of New Zealand in 2008.
Also 26, he has scored 10 tries in 17 starts for Quins to help them reach the top of the Premiership table.
Meanwhile, Foden is hopeful that Saints will again contribute a good number of players to the England squad for next month's tournament, after he joined Tom Wood, Chris Ashton, Courtney Lawes and Dylan Hartley at the 2011 World Cup.
"We're performing well and if guys are fit, hopefully we'll see a few more representing the green, black and gold," he added.
Italian Serie A side Atalanta have re-signed midfielder Marten De Roon from Middlesbrough for an undisclosed fee, 14 months after selling him to Boro.
[NEXT_CONCEPT]
In the UK, the May Day bank holiday offers a chance to take a short break away from the daily grind, and for some people that means hitching up the caravan to the back of the car and heading into the countryside.
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More than 70 people have been injured, four seriously, in an explosion caused by a suspected gas leak near the southern Spanish city of Malaga.
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Northampton Saints full-back Ben Foden believes his rivalry with Harlequins' Mike Brown can benefit both players and England.