Joining on a two-year deal, Bentley has established himself as a quality Super League second-row with his all-action style.
He made his mark with Bradford Bulls where he won the Championship Young Player of the Year in 2017 before joining Saints the following year.
The Ireland international, 23, started in last year’s Grand Final success but is currently sidelined after breaking a leg last month.
Bentley can also play centre or hooker and said: “I’m 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 want to make sure I play my part in helping Leeds win even more silverware in the years ahead.
“I’ve had four really enjoyable years at Saints and made some great mates for life. I’m 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.”
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 where 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’ll add greatly to the crop of talented forwards we have in the club.”
Warrington kept St Helens try-less and condemned them to a first home defeat of the season by grinding out a 6-2 victory.