The former St Helens and England half-back, 31, originally switched codes in 2011 but has now moved to Emerald Headingley on a two-year deal.
Eastmond shot to fame with Saints alongside current Rhinos assistant coach Sean Long and also represented England with Leeds director of rugby Kevin Sinfield.
He moved to Bath and converted to centre in union, making 72 appearances and scoring 16 tries, before a switch to Wasps in 2016.
Eastmond was capped six times by England between 2013 and 2014, including five starts, three of which were against the All Blacks.
He moved to join Leicester Tigers in 2018 but left Welford Road last summer.
Leeds last night revealed stand-off Robert Lui has been ruled out for up to three months with injury and full-back Jack Walker could miss the entire season.
Sinfield said, “We are delighted to be able to bring Kyle Eastmond back to Rugby League.
"With the injury to Rob Lui, we have been keen to boost our options in the half back position and I am sure Kyle will excel in our environment.
"We have been speaking over the last week and I know he had a number of options both in England and overseas to continue his Rugby Union career.
"The fact he is coming back to league is great news for us and the Super League competition.”
Rhinos head coach Richard Agar added: “I first worked with Kyle in the England Academy in 2006 and he has always been a naturally gifted half back. "He is an explosive runner and was part of a very successful St Helens squad.
"I know he will work well within our group and with our coaching team, especially Sean Long who he knows well from his days at Knowsley Road.
"Inevitably, there will be a period of time whilst he reacquaints himself with Rugby League after a decade in Union but I am excited to see how he develops in the coming weeks and months.”
Eastmond added: “I am excited to be coming back to Rugby League.
"Just like the timing was right for me when I went to Rugby Union, I think the time is right for me to come back to League now.
"I was coached by Richard Agar when I was younger and obviously played alongside Sean Long at Saints.
"This move is about me playing my very best rugby and I think this environment and this group of staff and players, gives me confidence that I can do that.
"If I’m happy and in the right environment then that means I can play at my best and I think that is what is going to happen.”