The 36-year-old centre-back is a product of the Chelsea's youth system, and has spent 22 seasons at the club.
He skippered his teammates to league success in 2005, that was the first of four top-flight successes, while he also won the UEFA Champions League with the west Londoners in 2012.
Part of his letter read: "It’s with overwhelming emotion that earlier this week the club and I announced this will be my last season as a Chelsea player.
"This has been the toughest decision of my life for me and my family?, but I always envisaged leaving on the right terms, in the right way and at the right time - and that is now.
"I feel I still have regular football in me but understand that opportunities here at Chelsea will be limited, it is therefore time for a new challenge."
He added: "With the backing and support of Mr Abramovich, we as players have been given the opportunity to fulfill all our professional desires, as this club moved into a new era where it matched our ambitions and became one of the biggest and most successful clubs in the world. For this I’d like to thank Mr Abramovich personally, and the board who have worked tirelessly to ensure the club continues to develop both on and off the field.
"For 22 years this club has been my life, it has given me the platform to compete against the best, break record after record, win trophy after trophy, and as we won more it only increased my desire to work harder and demand more from myself and the team. With this obsession I’ve been able to become Chelsea’s most successful captain and I’m immensely proud to be mentioned in the same company as Ron Harris, Peter Bonetti, Frank Lampard, John Hollins and many other greats as one of the highest appearance-makers for the club.
"Thank you to all my team-mates and managers over the years. To have gone shoulder to shoulder in many a battle with you has been an honour."