"Willian asked us for his [contract] termination," said Corinthians president Duilio Monteiro Alves.
"We are sad not to have him any more and also because it didn't go the way we expected. But we have to keep those here that are happy."
Corinthians have lost five of their past nine games, and Willian played in the second leg on Wednesday as the Brazilian side were knocked out of the Copa Libertadores quarter-finals by domestic rivals Flamengo.
"Whenever Corinthians lost and I didn't play well, my family received threats and insults on social media - my wife, my children, and recently they also started attacking my father, my sister," Willian told the Globo Esporte website.