The Romans, who last month appointed Paulo Fonseca as their coach, have also done two deals with Juventus -- signing defender Leonardo Spinazzola, while Luca Pellegrini joins the Italian champions.
Spinazzola, 26, has agreed to a four-year contract, with the club paying 29.5 million euros. Juventus have paid 22 million euros for defender Pellegrini, who is only 20 years old and spent last season on loan at Cagliari.
Roma Chief Executive Guido Fienga said that the club had been tracking Guinea international Diawara for a while. He is currently playing in the African Cup of Nations.
Fienga also said he had no doubts Italian international Spinazzola would soon prove his worth.
"Leonardo is a young player that has already acquired a lot of valuable experience in Italian football. I am certain that he will settle in here very quickly and will fit in well with the style our new coach implements," he said.
(Reporting by Arjun Panchadar in Bengaluru and Silvia Recchimuzzi in Gdynia; Editing by Peter Rutherford/Keith Weir)