The 25-year old arrives at the LNER Community Stadium on a two year deal, after four seasons at Championship rivals, Batley Bulldogs, where he made more than 100 appearances.
Buchanan brings versatility, being at home in the centre, wing or at full-back, and he also featured in the second-row for the Bulldogs last season.
He began his career at his home town club, Hull FC, where he made 13 Super League appearances before moving on to join Batley in 2021.
An international call-up from Scotland followed, with Buchanan featuring in all three of the Bravehearts' World Cup 2021 fixtures, against Italy, Australia and Fiji.
Coach, Mark Applegarth said: "I'm really looking forward to working with Keiran.
"He’s a player that’s played Championship for a number of years and he’s been there and done it all.
"I'm really happy to have a player of that sort of calibre joining us. He'll offer a lot of experience to the younger members of the squad and I’m really happy to be working alongside him."
Buchanan is the second new signing for 2025, joining Paul McShane.
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