Dani Ayala has put pen to paper on a new contract which will see him stay at the Riverside until 2020.
The Spanish defender, 25, signed for Boro from Norwich City in early 2014 following an initial spell on loan.
A towering presence in the heart of Boro's backline, Ayala won a string of plaudits on the club's journey to promotion from the Championship last season, including the PFA Championship Fans' Player of the Year.
After signing his new deal, Ayala said: "I didn't think about anything else. I only had one decision in my mind and that was to sign with Middlesbrough.
"When I wasn't at my best Boro gave me the chance, and I've improved myself under Aitor Karanka.
"All of the fans really like me and I'm really settled here.
"My family are really happy here, my son was born here and hopefully my second will be born here too!"
With the contract signed and sealed, Ayala can't wait to get started in the Premier League.
"After two hard seasons in the Championship, I can't wait. It's a dream come true," he said.
"Hopefully, with the signings we've made we can do well next season."