The 26-year-old was brought to the Stadium of Light during the summer with the intention of being the first-choice central defensive partner of skipper John O’Shea.
But Diakite made just three Premier League appearances during Gus Poyet’s tenure – all of which came when Wes Brown and O’Shea were ruled out through suspension and injury respectively.
Fiorentina, currently lying in fourth spot in Serie A, beat off competition from a number of Italian clubs to take Diakite on loan until the end of the season.
And after making his debut in Wednesday night’s 2-1 Coppa Italia semi-final first leg defeat to Udinese, Diakite is pleased to have returned to Italy after ending his seven-year stint at Lazio last summer.
Modibo Diakite: “Fiorentina have given me so much, now I have to pay back the confidence they have shown in me.
“Under (Paolo) Di Canio, I was playing at Sunderland, but then I only had a small role under Poyet, who played Brown and O’Shea.
“When the call came from Fiorentina, I immediately wanted to come here.
“The Viola are a great club. We think we can get to the final of the Coppa Italia. We also have the chance to get third place in the league.”