The forward, who scored 21 tries in 25 appearances for Oldham this term before they were relegated from the Championship, has signed a two-year deal with the Airlie Birds.
Although he has been operating in the lower leagues, Langtree does have some experience at a Super League club.
He was a promising second-row in St Helens’ academy when, at just 17, he was banned for two years after testing positive for cocaine.
Langtree returned to action with Oldham where he won the Championship One Player of the Year award in 2014 before establishing himself as one of the finest forwards in the second tier.
Radford - who has also signed Salford Red Devils prop Levy Nzoungou for 2019 - said: “I’m really pleased to have Danny on board because I feel that he has the attributes to be a great addition to our squad.
“There aren’t many forwards across the game with a try-scoring record like he has, so I’m really excited to see how he goes for us.
“We have been open about adding some depth to our squad and we still have one or two areas we would like to improve before the new season comes around." Langtree said: “I’m really excited to be joining a big club like Hull and I’m really looking forward to a new experience.
“I’m aware of the step up in quality to Super League, but I’m looking forward to testing myself against some of the best players in the division and at a great club.
“I’ve come here to be in the first team every week. My target is to play as many games for Hull FC as I possibly can and I’ll work as hard as I can to show I can do that.”