Napoli club president De Laurentiis on Friday confirmed the deal via Twitter with this message: "And welcome to you too, Diawara."
Diawara, 19, who started his career as a player for San Marino Calcio in the third division, made an impression last season as Bologna finished 14th in the Italian top flight, making 34 league appearances.
The 19-year-old refused to attend pre-season training with Bologna, as he tried to force a move elsewhere.
Valencia, Roma, Leicester City, Aston Villa and Juventus were among the clubs interested in the Guinean.
His ascent to Serie A was extremely rapid, joining Bologna from San Marino in 2015 summer.
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