On May 11, 2008, Dubai announced that Bosingwa had come to verbal agreements with Chelsea for a €20.6 million / £16.3m transfer. He put pen to paper on a three-year deal, and joined the London club after Euro 2008, along with Portugal national team boss Luiz Felipe Scolari and fellow international Deco. On July 16, 2008, Bosingwa was presented with the number 16. He was handed the number 17 for Chelsea on August 8, 2008, swapping with Scott Sinclair.

Bosingwa made his Champions League debut against Bordeaux on September 16, 2008, along with his fellow team mate Deco. He scored his first goal for Chelsea against Stoke City on September 27, 2008. He scored again for Chelsea with a stunning left-footed goal against West Brom from 25 yards.

On February 1, 2009, Bosingwa was involved in an incident on the field where he kicked Liverpool player Yossi Benayoun directly in the back, without making any attempt to play the ball. Although the referee Mike Riley did not see it, assistant referee Mo Matadar would later claim to have seen the incident, he signaled a foul at the time but then awarded Chelsea a throw-in following the incident. In the days following the match, the FA claimed that FIFA regulations prevented them from imposing disciplinary sanctions as no booking or sending off was issued.

Bosingwa was banned for three European matches as a result of comments he made about the referee on Portuguese television, following Chelsea's clash with FC Barcelona on 17 June 2009 in the second-leg of the semi-final of the Champions League, which resulted in a draw. The ban was later reduced to two matches on appeal.

On June 19, 2009, Bayern Munich chairman Karl-Heinz Rummenigge conceded defeat in his club's attempt to sign the Portuguese defender, making it all the more likely that he will stay in London at Stamford Bridge.