Roberts, 20, enjoyed a stellar season for the Bhoys in 2016-17, scoring 11 goals in 47 appearances as the Scots waltzed to a domestic treble.
And the winger will now have another chance to shine at Parkhead as he prepares for another stint north of the border.
"Welcome back Paddy!" Celtic wrote on their Twitter page as the season-long move was confirmed.
The Citizens also wished their youngster "good luck" ahead of the loan, which made him Celtic's fourth arrival of the summer window and sees him join ex-City man Olivier Ntcham in Scotland.
Roberts signed for City from Fulham in 2015 for an undiclosed fee believed to be worth around £12 million.
The youngster, however, has struggled to break into the squad at the Etihad Stadium, making just three appearances for the club before joining Celtic in his first loan move in January 2016.