The 29-year-old became available after he went unsold in the Indian Premier League (IPL) player's auction for the upcoming 11th edition of the tournament.
Ecstatic to have Ishant in the team, Sussex's director of cricket Keith Greenfield said that securing the Indian fast bowler's services would be crucial for them especially in the absence of Jofra Archer and Chris Jordan.
"Securing Ishant's services has been very important for us. Once Jofra and Chris were picked up in the IPL draft, it became crucial to sign an experienced international-quality seam bowler who could provide valuable support and know-how to the rest of the seamers," he said.
"Ishant has the attributes to be very successful in early season English conditions and a great role model. He fits the bill perfectly and we look forward to welcoming him to Sussex Cricket," Greenfield added.