City have reached an agreement worth €65 million ($71m) for the Belgium forward, who is now set to travel to Manchester to undergo a medical.
The 21-year-old will arrive at the Etihad Stadium as a replacement for Riyad Mahrez, who moved to Al Ahli earlier this summer.
Doku, who will be the club's summer third signing after Mateo Kovacic and Josko Gvardiol, has scored 12 goals in 91 games for Rennes since arriving from Anderlecht in 2020. He has been capped 14 times by Belgium, scoring twice, and was part of the squad for the 2022 World Cup.
After securing Doku, City will continue their search for a midfielder to fill the void left by Kevin De Bruyne. The 32-year-old has been ruled out until the new year after undergoing surgery on a hamstring injury.
Centre-back Aymeric Laporte is also close to joining Al Nassr after the Saudi Pro League side agreed a fee with City. Both Laporte and Cancelo were left out of the squad ahead of the 1-0 win over Newcastle United on Saturday.