15th May 2018, 16:33 | Widnes
The 27-year-old, who is out of contract with the Rhinos at the end of the season, made just two appearances for the Vikings during his initial month-long deal after being unable to play in the Ladbrokes Challenge Cup.
"I've really enjoyed my spell at the club and I am excited to have this opportunity to continue to represent the Vikings further into this season," said Keinhorst, who is set to face Betfred Super League leaders St Helens at the Dacia Magic Weekend on Saturday.
"I'm looking forward to building some consistency, supporting the team to pick up points and climb the table."
Widnes coach Denis Betts said: "We are grateful to Leeds for extending Jimmy's loan spell at the Vikings.
"Jimmy is a quality, experienced player, who adds real threat to our edges."
Leeds have an option to recall Keinhorst.