The veteran midfielder comes off contract at the end of June and there's been little movement from the club's side about discussing new terms.
In a post to social media, De Bruyne announced his decision, with what appeared no co-ordination with City's media team.
The Belgian declared: "These will be my final months as a Manchester City player.
"Nothing about this is easy to write, but as football players, we all know this day eventually comes. That day is here - and you deserve to hear it from me first.
"Football led me to all of you - and to this city.
"Chasing my dream, no knowing this period would change my life.
"This city. This club. These people - gave me EVERYTHING!
"I had not choice but to give EVERYTHING back.
"And guess what - we won EVERYTHING!
"This will always be our HOME.
"We cannot thank the city, club, staff, teammates, friends and family enough for this ten-year ride.
"Let's enjoy these last moments together."
De Bruyne has been in talks with new MLS franchise, San Diego FC, as confirmed by club officials last week. It also emerged in recent days that Saudi club Neom SC are chasing the veteran's signature.