The Greece international had been close to joining Premier League side Arsenal but will instead replace Bayern Munich-bound Mehdi Benatia at the Stadio Olimpico
The 23-year-old defender had been on the verge of joining Arsenal but, as revealed by Goal, the Gunners' hopes of securing him collapsed when the Giallorossi offered more than the £8m that Arsene Wenger's side were willing to pay.
"Konstantinos Manolas is now a Roma player," Roma announced on their official Twitter account on Tuesday.
The Italian side were in need of a replacement for the Bayern Munich-bound Mehdi Benatia and they agreed a fee with Olympiakos - which could could rise to €15m (£11.9m) 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.