Anfernee Dijksteel has signed a new four-year-and-a-half-year contract at Boro.
Dijksteel, 24, has been an outstanding performer for Boro under Neil Warnock and is a league ever-present so far this season.
The Dutch defender signed from Charlton Athletic in the summer of 2019. Recovering from a long-term injury lay-off, Dijksteel returned to the starting line-up under Warnock in July and helped Boro away from the relegation zone.
His form has gone from strength-to-strength in a side which has conceded just four Championship goals so far this campaign.
The new deal sees Dijksteel committing his future to the Boro until 2025, news which has pleased manager Neil Warnock.
"I'm absolutely delighted," said the boss. "He really deserves it.
"He's worked ever so hard since I came here, and I've been really pleased with the improvement he's shown in his overall game.
"He's a great signing for Middlesbrough for the next few years."