The Brazilian international, who turned 32 on Sunday, was keen to extend his stay at Stamford Bridge by three more years but Chelsea were only interested in offering him a two-year contract.
Willian is keen to remain in London however, and Arsenal have jumped at the chance to offer him the three-year contract he craves.
Willian joined Chelsea from Russian side Anzhi Makhachkala in the summer of 2013 and won two Premier League titles, one League Cup, one FA Cup and the Europa League during his seven seasons in West London.
In an open letter to Chelsea fans on Sunday, the former Shakhtar Donetsk man bid farewell to the club ahead of his impending departure.
"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," Willian wrote.
"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.
"Yet, beyond the trophies, I learnt a lot about myself. I developed a great deal, becoming a better player and a better person. With each training session, with each game, with every minute spent in the dressing room, I was always learning.
"I am really grateful to the Chelsea fans for the affectionate way they welcomed me at Stamford Bridge and their support throughout my time at the club. There was also criticism, which is normal, what is important though is that both the affection and criticism drove me to always give my all in every training session, every game, to be constantly improving until my very last minute in a Chelsea shirt!
"The time has now come to move on. I am certainly going to miss my team-mates. 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!
"My heartfelt thanks go out to all of you and God bless you!"
