The left-arm spinner, who has taken 203 wickets in 59 Test matches, will play first-class games in the first two months and then T20 Blast matches as the Proteas have no fixtures scheduled during that period.
Maharaj will be one of two overseas players at the start of the season for Leicestershire after being promoted to the first division, along with captain and Australia batter Peter Handscomb.
The 35-year-old Maharaj has previously featured in county cricket with Lancashire and Yorkshire.
"I've been digging into the history of the club, and it seems like a very exciting time to sign," Maharaj said. "The boys have done some exceptional work throughout the season and have built some great momentum heading into next year. It's early days, but we're tracking in the right direction, so who knows how far this team can go?"
Leicestershire's director of cricket, and former Proteas left-arm spinner Claude Henderson said: "It's fantastic to welcome Keshav to Leicestershire. Alongside his exceptional cricketing abilities, he will bring vast international experience, leadership skills, and a huge passion for the game - all of which will be fundamental to our changing room."
Another former Proteas international, Alfonso Thomas, is the current head coach of Leicestershire.
Quinton de Kock produced a batting masterclass as South Africa crushed Pakistan by eight wickets in the second ODI in Faisalabad, chasing down 270 with nearly 10 overs to spare to level the three-match series 1-1.
The Proteas fought to the bitter end but ultimately fell to a two-wicket defeat with two balls to spare against Pakistan in the first ODI in Faisalabad on Tuesday.
Dewald Brevis has been ruled out of the Proteas' three-match ODI series in Pakistan, that starts in Faisalabad on Tuesday, after suffering a low-grade shoulder muscle strain.
Pakistan cruised to a four-wicket victory against South Africa in the third T20 International at the Gaddafi Stadium on Saturday to claim the series 2-1.
