On 23 August 2007, Chelsea officially announced that Belletti had signed a three-year contract after the player agreed personal terms and passed a medical. He made his Premier League debut for Chelsea on 25 August, when he came on as a substitute for John Obi Mikel in the 64th minute of their home game against Portsmouth, with the game ending 1–0 to Chelsea.

He made his first start for Chelsea against Aston Villa at Villa Park. Despite a strong debut, Chelsea lost the game 2-0. Following the departure of José Mourinho, Belletti became a regular starter in the right-back position for Premier League, Champions League, and Carling Cup games, facing competition from Paulo Ferreira and Michael Essien for his place in the team. In his first season at Chelsea, Belletti scored two goals, both shots from long-range, the first away at Wigan Athletic in November 2007, and the second at Stamford Bridge against Tottenham Hotspur in January 2008. The latter was eventually voted by Chelsea supporters as "Chelsea Goal of the Season" in 2007–08.

José Bosingwa's arrival at Chelsea at the start of the 2008–09 season caused Belletti's relinquishment of the right-back position at Stamford Bridge. However, Luiz Felipe Scolari, gave Belletti playing time as a utility player, providing valuable back-up in the central midfield, defensive midfield, and right wing positions. His long-range shots have given him cult status among the Chelsea support. In October 2008, he added to his collection of spectacular goals with a powerful shot from 30 yards, in to the top corner, against Middlesbrough at the Riverside, for his fourth league goal for the club. Belletti scored his third goal of the 2008-09 season against Stoke City, scoring the leveling goal before teammate Frank Lampard scored the winner in the dying seconds.

He returned from an achilles injury to make his first appearance for Chelsea in the 2009–10 season against Burnley on 29 August. He replaced Bosingwa late in the second half.