Boro have agreed a fee of around £4m for the 24-year-old forward, who becomes the club’s second signing of the summer following the arrival of Myles Peart-Harris.
The Teessiders agreed a deal for Joseph at the start of the month, with the attacker travelling to Rockliffe Park to tie up the deal.
However, an issue emerged that meant Boro broke off their discussions with Hull City, who initially indicated that Joseph would be returning to the MKM Stadium.
Boro’s interest remained intact though, and a further round of talks last week resulted in a new agreement being brokered.
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“I’m so delighted to sign for this massive club,” said Joseph, after his move was finally completed. “It’s such an ambitious club and I really want to help it achieve its goals.
“I’m an all-action centre forward, but most importantly I want to score goals and help the team win games.”
After progressing through the academy at Wigan Athletic, Joseph joined Swansea City in 2021 and had loan spells at Cheltenham Town and Oxford United.
He moved on to Blackpool in 2023, making 66 appearances for the Seasiders before heading to Hull in a £2.5m move in January 2025.
He was an important part of Hull’s promotion success last season, scoring eight goals and claiming five assists in his 44 senior appearances.
He started the second leg of Hull’s play-off semi-final win over Millwall at the Den, but suffered an ankle injury that meant he was forced to sit out the Tigers’ victory over Boro in the final at Wembley.
He continues to work his way back to full fitness, but Boro officials are confident the injury is not a long-term concern.