Keinhorst has not played for Leeds since suffering a knee injury, which required minor surgery, in their defeat at Widnes on February 25.
He is now fit and available, but has not been selected for either of Rhinos’ past two matches, due to Ash Handley’s form at centre.
The loss at Widnes was only his second Super League appearance this season, though he also played in the World Club Challenge defeat at Melbourne Storm.
He scored a hat-trick in the win over Hull KR at Elland Road two months ago, which took his tally of tries for Leeds to 30.
Keinhorst - who is out of contract at the end of this season - made his debut in 2012 and has started 52 games for Rhinos, with another 27 appearances off the bench.
Rhinos’ supporters’ player of the year two years ago, he said: “I’m excited to be linking up with Widnes Vikings on this loan spell.
“I have found my opportunities limited at Leeds recently since my knee injury and I’m eager to play regularly and show what I can do.
“I’ll be joining Widnes at a time when they are a little short with injuries, so this is a really positive opportunity for all parties.
He added: “Widnes are a team I’ve got real respect for.
“I’ve enjoyed playing against the Vikings in the past – they’re a tough team and have great supporters.
“I am looking forward to working hard for the team and hopefully helping them to pick up points in the upcoming games.”
Rhinos coach Brian McDermott insists the move does not mean the end of Keinhorst’s Rhinos career.
“Jimmy is at the stage of his career where he needs consistent first team rugby and needs to be starting most weeks,” he said.
“If he is not getting that with us he needs to get it somewhere.
“We have put him on loan to Widnes to benefit us and Jimmy so when he does get that call up to our side he has played recent Super League.
“He is a very popular player and a very good player and he certainly has a future here.”
Denis Betts, Widnes’ coach, said he is grateful to Rhinos for allowing the loan move.
“Jimmy is an outside-back and utility-player who offers solid experience and proven quality,” he said.
“Having lost a number of our outside-backs to injury recently, his addition to the squad will give us a real boost.”
Keinhorst will join Widnes on Monday, but will not be eligible for their Ladbrokes Challenge Cup tie against Coventry Bears next weekend to avoid him becoming Cup-tied.