The back-rower, who is from Leeds and played his junior rugby with local community club Oulton Raiders, will join Rhinos next season on a two-year deal.
A Leeds fan, he is Rhinos’ first addition for 2022 and was announced just 24 hours after director of rugby Kevin Sinfield confirmed he is leaving the club.
Bentley was a starting second-rower for Saints in last year’s Grand Final win over Wigan Warriors, but is currently recovering from surgery on a broken leg suffered against Leigh Centurions last month.
“I am looking forward to joining the Rhinos for next season,” Bentley, an Ireland international, said.
“I am a Leeds lad and grew up supporting the team, sitting on the South Stand wall and dreaming of running out on the pitch as a Leeds player.
“I have had a taste of success at St Helens and I want to make sure I come to Leeds and play my part in helping the team win even more silverware in the years ahead.”
Bentley began his professional career at Bradford Bulls in 2016, having originally been part of Featherstone Rovers scholarship programme.
He made the first of his 28 appearances for Bulls that season and won their young player and players’ player of the year awards, as well as being named the Championship’s young player of the year, before joining Saints on a three-year contract in 2018.
After a spell on dual-registration at Sheffield, Bentley made his Saints debut at home to Hull FC three years ago, the first of three appearances that season.
He featured 17 times in 2019, playing at hooker, back-row and centre, when current Rhinos assistant-coach Sean Long was on Saints backroom staff.
He was named Saints young player of the year at the end of last term, having scored five tries in 19 appearances.
Rhinos coach Richard Agar said: “We are delighted to have secured James’ signature.
“He is a young British forward who has developed tremendously well over the last couple of years, when his form and achievements for St Helens have been outstanding.
“He is a real competitor, with a strong attitude and desire.
“He is a local lad who will add greatly to the crop of talented forwards we have in the club.”
Bentley added: “I have had four really enjoyable years at Saints and made some great mates for life
“I am doing everything possible to get back healthy and on the pitch as soon as possible and hopefully I can finish the year on the best possible note with St Helens.”
Speaking after Bentley’s injury, Saints coach Kristian Woolf described him as “a really respected member of our team”.
He said: “He has been a massive part of our team and success over the last few years, especially the back end of last year.
“He is a player who gives his all every week.”
Bentley’s signing will increase speculation second-rower Rhyse Martin is set to leave Rhinos when his contract expires this autumn.