Eden Hazard, who previously wore the number at the Europa League holders, departed for Real Madrid in the off-season.
Hazard spent seven years at Chelsea after joining from French side Lille in the summer of 2012, winning two Premier League titles, two Europa League trophies, one FA Cup and one League Cup.
Hazard had been wearing the number 10 jersey since the start of the 2014/2015 season, taking over from Juan Mata, who had left for Manchester United.
Willian has worn the number 22 jersey since joining the club from Anzhi Makhachkala in the summer of 2013, but will now benefit from Hazard’s departure.
The 30-year-old recently donned Brazil’s number 10 jersey, helping the Selecao to Copa America glory on home soil.
Speaking to Brazilian TV show Desimpedidos, Willian claimed: ‘I was offered the number 10 jersey ahead of next season and I will take it.
“I have always liked playing with number ten.
“Since I was a kid playing indoors football, I have always used the number ten. At Corinthians I was also number ten.”
New-signing Christian Pulisic and Callum Hudson-Odoi were also contenders to take over from Hazard but have lost out to Willian.
Chelsea stole Willian from the clutches of Tottenham Hotspurs, where he had undergone a medical, for a fee of £30m from Russian side Anzhi Makhachkala in the summer of 2013.
Willian has won two Premier League titles, one League Cup, one FA Cup and recently added the Europa League to his trophy collection in London.