The former Hull FC, Wakefield Trinity and France head coach has been drafted in to replace first-team coach James Lowes, who is thought to be heading back to rugby union with Ealing Trailfinders.
Agar, 47, joined Leeds in the autumn, but in the new role of head of player and coach development having left a similar job at NRL club St George Illawarra.
Rhinos head coach Dave Furner, though, has endured a troubled start to his own reign in the top job, the club losing eight of their opening 10 games to be left bottom of Super League.
Lowes – who has had two spells as Yorkshire Carnegie head coach – is understood to have handed in his notice last Thursday, before that night’s shambolic 45-26 defeat at Hull KR. A former half-back with Featherstone Rovers, Widnes and Dewsbury Rams, Agar coached France at the 2013 World Cup and also spent time as Warrington coach working under former Leeds chief Tony Smith before heading to Australia at the end of 2017.
He left St George-Ilawarra after just one year of a three-year deal due to personal reasons and the new Leeds director of rugby employed him in the changes that also saw Furner brought in from South Sydney.
Lowes, who returned to the Rhinos last summer after Brian McDermott’s sacking as head coach, is also part of England head coach Wayne Bennett’s backroom staff so that will now create a vacancy in the international set-up.
Ealing – where London Broncos play – are one of the most ambitious Championship clubs with Premiership aims.
In a statement, Sinfield said: “On behalf of everyone at the Rhinos I would like to thank Jimmy for his efforts at the club.
“I know he is excited about the new challenge ahead of him and he leaves with our best wishes.
“In the interim, Richard Agar will join the first-team coaching staff combined with his role as head of player and coach development. Richard is a vastly experienced coach and will be a great addition to the coaching staff.”