Chelsea defender Tomas Kalas will spend the 2015-16 season on loan at Middlesbrough.
Kalas, who has previously spent time on loan at Vitesse Arnhem and Cologne, returns to Boro after making 17 appearances for the Championship side earlier this year.
Boro, managed by Jose Mourinho's former Real Madrid assistant Aitor Karanka, had also taken Kenneth Omeruo and Jamal Blackman on loan from Chelsea last season.
"I'd already decided at the end of last season that if Chelsea would like to get me on loan again this would be my first choice, so it was easy when I heard I could stay," Czech Republic under-21 international Kalas told Boro's official website.
"I think we're still building the team. It's a mix of everything. Everyone is still working on it and I think we will be stronger than before."
READ: Tomas Kalas - "We'll be even stronger this season." http://t.co/iJiReM41Jr #BoroInSpain #UTB pic.twitter.com/htU6m0eRD6
- Middlesbrough FC (@Boro) July 17, 2015
Middlesbrough, who also signed Stewart Downing from West Ham this week, are looking to secure promotion back to the Premier League after losing last season's playoff final.
Kalas was unable to play a part in the playoffs because his loan spell had already ended, but he said: "Obviously I wanted to be involved.
"Those were something special at the end because you were playing for promotion and I didn't have a chance to help the team. It was a bit disappointing I couldn't play, but this is a new challenge and hopefully we can get promoted without the playoffs."