The 20-year-old forward has put pen to paper on a four-and-a-half year contract at the Riverside after [Boro](http://www.thenorthernecho.co.uk/sport/football/middlesbrough?ref=au) agreed a £1m deal with Chelsea.
Castledine travelled to Teesside on Thursday evening to put the finishing touches to the move, which has now been confirmed.
Boro are understood to have beaten a number of Championship clubs to the capture of the versatile forward, who has dazzled on loan at Huddersfield Town in League One in the first half of the season.
"I’m absolutely over the moon to join what is a massive, massive club," said Boro's new recruit.
"It’s a really big step for me in my career and I’m very excited to be here.
"There’s always been trust in young players at this club if they can perform, and I want to perform at my highest level.
"I’m fit, healthy and I’ve been playing well, so I feel ready to go. I want to help the team as much as I can from the first day."
After Chelsea recalled Castledine before his switch to Boro was confirmed, Huddersfield boss Lee Grant said: “We were not at all surprised by the level of interest shown in securing Leo’s services permanently, and of course the decisions were inevitably out of our control.
"Having worked in both the summer and winter windows to explore the potential of signing Leo permanently, we fully respect and understand his and Chelsea’s decision for him to test himself in the Championship this season."
Castledine, an England youth international, spent time out on loan at Shrewbury last year before continuing his progression with Huddersfield, where he scored 12 goals - including several stunners - and provided five assists in the first half of the season.
He made his Chelsea debut in the Carabao Cup semi-final second leg victory against Boro two seasons ago.
Speaking about Castledine in his Friday afternoon press conference before the deal was confirmed, boss Kim Hellberg said: "He's a good player. He's done a brilliant job at Huddersfield, doing a lot of good stuff. He's scored a lot of goals. He's a good player."
Boro will look to further strengthen their attack after the arrival of Castledine, with Jeremy Sarmiento one of several targets.