Zappacosta is the third Serie A star Chelsea sign under Antonio Conte.
The wing back signed a four-year deal.
Zappacosta was selected by Antonio Conte as a reserve player for Euro 2016, but he did not get to make his Italy debut until November, by which time Conte was at Chelsea.
Chelsea made one confirmed signing before tonight's 11pm transfer deadline expired - that of Italian wing-back David Zappacosta, 25 from Torino.
Zappacosta has signed a four-year contract with the Premier League champions and the Italy global is delighted to have made the move, explaining playing for Chelsea in the English top flight is a dream come true.
Zappacosta has impressed with Torino and with the Italian national team, though is not the big name many Chelsea fans will have been craving.
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Chelsea have a paper thin squad following the departure of a number of key players, and the Blues need to recruit heavily to prepare a squad who can help the reigning Premier League champions challenge for top honours once again this season.
A deal for Alex Oxlade-Chamberlain was agreed with Arsenal but the England worldwide didn't want to play for Chelsea.
Meanwhile, Leicester City midfielder Danny Drinkwater is set to undergo a medical ahead of sealing his last-hour move to Chelsea.
"I hope I can show my quality to the coach, my team-mates and the supporters", he added.
The former Atalanta man has been described as a strong crosser of the ball, making him a useful outlet from full-back and ideal to fit into Conte's 3-4-3 system.