The 33-year old moves to the subcontinent after his contract with Turkish Super Lig side Kayserispor expired on July 1.
This is the 10th club he will be representing in his professional career, having had stints in home country Ghana with Liberty Professionals, as well as Italy, England, France, United Arab Emirates, China and Turkey.
"I am extremely excited to start this new chapter in my career," Gyan told the media.
"I have heard great things about NorthEast United FC and the Indian Super League and I am thrilled to be a part of it for the upcoming season.
"I am confident that we can better the wonderful season the club had last year."
Gyan is Ghana's leading top-scorer with 51 goals from 107 International matches. He is also the country's leading scorer at the Africa Cup of Nations and Fifa World Cup.
The Highlanders finished fourth in the ISL last season, their highest ever placing since their formation in 2014. They kickstart their 2019-20 campaign with an away tie at Bengaluru FC on October 21.