Leaning will leave Headingley at the end of the season after 15 years with the White Rose club.
Leaning, 25, has struggled to hold down a regular place and hopes the move will resurrect his career.
“I’m really excited by the prospect,” he said. “I think it’s at the right stage of my career to have a little bit of a change, and hopefully it all works out to be a positive one.
“It’s a big move lifestyle-wise, and that did cross my mind at first. But, since then, all that’s come back to my mind is excitement as opposed to any nerves.
“For me, that’s one of the reasons I chose Kent. It was a feel thing and a gut instinct. It felt like the right fit for me having spoken to the club. I’m really looking forward to it.
“I leave Yorkshire with nothing but fond memories. I’ll be giving it my all towards the end of the season, and there’s still two bits of silverware on offer if things go well.
“It’s my childhood club - I’ve been here since I was 10-years-old. It will be a change, obviously, but I’m really excited.”
Leaning, born in Bristol, played age-group cricket for Yorkshire up to U-17 level before graduating to the Academy side.
He has played 68 first-class matches for the club, scoring 2,955 runs at 30.46.
Martyn Moxon, Yorkshire’s director of cricket, said: “He’s been a great model professional during his time here and has been part of two Championship-winning teams.
“He’s had a good career, but hasn’t played regular first-team cricket. At this moment in time, the opportunity to play first-team cricket is exactly what he needs.
“We thank him for representing us with pride and wish him every success in the future. He has contributed well to Yorkshire County Cricket Club over the years.
“At this stage in his career, second XI cricket is no good for him and a move to another county, who can guarantee him first-team cricket, is the right thing all round for his future.”