The Colombia international had been on loan at Spanish side Girona, where he made just four appearances in six months, reports Xinhua news agency.
Moreno arrived in Rio de Janeiro for a medical on Saturday and took part in his first training session on Monday, Flamengo officials said.
Manchester City signed Moreno in August 2016 from Colombia's Atletico Nacional but the 21-year-old is yet to make his debut for Pep Guardiola's team. He has been capped eight times for Colombia and scored one goal.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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