George Friend has penned an extended deal with Boro which will keep him at the Riverside until 2020.
A firm favourite with Boro fans since singing from Doncaster Rovers in 2012, Friend has been named in the PFA Championship Team of the Year for the last two seasons running.
The defender follows teammate Dani Ayala in committing his future to the club, after the Spaniard signed an extended contract last Friday.
Friend said: "For me committing to the Boro for another four years is because of so many things - a fantastic chairman, a brilliant manager and staff, great teammates, absolutely amazing fans and a beautiful area in which to live.
"I am very grateful and thankful to everyone who has encouraged and supported me.
"What I can promise in return is to continue to try to give 100% commitment to the club.
"My grandfather taught me many things, one of which was to play every match with the same energy and focus as if I was playing in the world's biggest stage.
"This is what I've always tried to do from the beginning of my career to now, as we stand on the threshold of the Premier League.
"In my eyes the Riverside is the best and biggest stage and to be given the opportunity to extend that privilege is everything!"