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The 25-year-old midfielder, who had played for Sao Paulo since 2015, recently extended his contract until 2021.
In 147 games he scored 12 goals and contributed 11 assists.
At Marcelo Bielsa's Lille he will join countryman Luiz Araujo, a 21-year old striker who arrived on June 14 from Sao Paulo.
"Since I was a little boy, playing in Europe has always been a dream for me," said Mendes.
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"Today, I am very happy to be at Lille. I also want to grow individually and collectively, especially under Marcelo Bielsa who has a great reputation."
Lille's director general Marc Ingla added: "It's a new Brazilian in the squad, which will bring even more smiles to an already cheerful dressing room."