The French striker joined the Bhoys on a four-year deal from Fulham on Tuesday and will now link up with ex-Cottagers teammate Roberts at Parkhead.
19-year-old winger Roberts joined the Scottish champions on an 18-month loan deal in January so will get the opportunity to spend a further 12 months playing with Dembele, this time in Glasgow.
He took to Twitter to show his delight at the signing of Dembele:
Big signing!! Good to play alongside you again bro.. Welcome https://t.co/tIOymvDNw8
— Patrick Roberts (@patrick7roberts) June 28, 2016