Kelly has accrued 3,151 runs in 110 List A games and is approaching 2,000 runs in T20 matches with a strike rate of 127.
He is currently taking part in New Zealand’s white-ball tour of Bangladesh.
“Nick is a high-quality signing for us and someone we’re really excited to bring into the group,” head coach Alfonso Thomas said.
“His record across all formats is excellent, particularly in first-class cricket where nearly 5,000 runs at close to 40 shows his consistency and class.
“He’s a versatile batter who can adapt to different situations, and his experience in white-ball cricket will be a real asset for us in the One Day Cup and T20 Blast.”
Leicestershire have drawn two and lost one on their return to the first division of the County Championship this year and take on reigning champions Nottinghamshire at Grace Road on Friday.
