The ex-Everton player, who began his career with Leeds United and also spent time with Aston Villa, had been without a club since his contract with the Toffees expired in June.
He has now revealed he will hang up his boots, penning a farewell statement on social media.
"So today I announce my retirement with nothing but happiness and gratitude for what this beautiful game has given me," Delph wrote.
"[A] massive shout out to my family, friends, players and staff from the amazing clubs I have been fortunate enough to represent as well as the support I have had from the fans."
Across a 16-year career, Delph was a two-time Premier League winner with City, while he was a runner-up in the FA Cup and Carabao Cup with Villa.
Capped 20 times for the Three Lions, he was a surprise recall for Gareth Southgate after three years away from the international game in 2018, before being part of the squad that came fourth at Russia 2018.