The Brazilian winger, 32, confirmed his departure Sunday, waving goodbye to the club and its fans with a heartfelt letter on social media.
"They were seven wonderful years. In August 2013, when I received the offer from Chelsea, I was convinced that this was where I had to play. Today, I am certain that it was the best of decisions. There were so many happy times, some sad, there were trophies, and it was always very intense," he wrote.
"The time has now come to move on," Willian added, without making mention of his next stop. "I am certainly going to miss my teammates. I will miss all the staff at the club who've always treated me like a son and I will miss the fans. I leave with my head held high, safe in the knowledge that I won things here and always did my best in a Chelsea shirt!"
Confirmation of Willian's departure comes just one day after the Blues' prolonged 2019-20 campaign came to an end with a Champions League defeat to Bayern Munich.
Chelsea were apparently unwilling to offer the veteran winger a new multi-year contract, even though manager Frank Lampard was believed to be interested in retaining his experience and creativity.
In his seven years at the club, Willian won two Premier League titles, one FA Cup, and a Europa League crown. In 36 league appearances this past season, he scored nine goals, adding seven assists.