The 23-year-old spent last season on loan at Craven Cottage, scoring six times in 33 matches as Fulham lost in the Championship play-offs.
He has played for five clubs on loan, including Reading, since his last Chelsea appearance in December 2012.
"It felt like I had to come back and help the team reach what we were chasing last year," said Piazon.
"This is the first time I will spend two seasons with a club. I am excited to see the boys and get back into training."
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Fulham have re-signed Brazilian forward Lucas Piazon on a season-long loan from Premier League champions Chelsea.