The Greece international had been close to joining Premier League outfit Arsenal but he will instead replace Mehdi Benatia at the Stadio Olimpico.
The 23-year-old defender had been on the verge of joining Arsenal but the Gunners' €10 million bid was rejected, thus allowing the Giallorossi to pounce.
"Konstantinos Manolas is now a Roma player," read a Tweet posted on the Serie A side's official Twitter profile on Tuesday evening.
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Roma was in need of a replacement for the Bayern Munich-bound Mehdi Benatia and agreed a fee with Olympiakos - which could could rise to €15 million depending on bonuses - just hours after the Moroccan completed his move to the Allianz Arena.
The Greece international, who could make his debut in Saturday's Serie A opener against Fiorentina at the Stadio Olimpico, has penned a five-year contract with Rudi Garcia's side.