The Spanish has reportedly sign a two years deal and will pocket £50,000 per week at Roma, who have the option to extend the contract by another 12 months.
The 32-year old departure will bring an end to his five-year spell in the Premier League, following his arrival in August 2015 from Barcelona.
Since joint the Blues, the La Masia graduate has scored 43 goals in 201 games, and has helped the West London base club win the Premier League in 2017, FA Cup in 2018 and Europa League in 2019.
The 2010 World Cup winner with Spain contract was due to expire at the end of the month, but he signed a short-term extension earlier this week to allow him to end the campaign with the Blues.
However, since the arrival of Frank Lampard as Chelsea boss, the Spanish attacker has seen his playing time decrease, featuring 18 times.
With the Blues already agreed deals to sign Ajax’s Hakim Ziyech and RB Leipzig’s Timo Werner at the end of the season.
Both signings giving Frank Lampard options on the attack line ahead of next season.