The under-20 international played in the London club's pre-season tour match against Barcelona in the United States having been granted permission by his Brazilian team.
"I am very happy to complete my transfer," Kenedy told the club's official website. "It is an exciting time for me to join ... and I hope I can become an important player for the club.
"I very much enjoyed my time with the squad during pre-season," added the teenager who can play on either flank or down the centre.
Kenedy's acquisition could signal the departure of Nigeria winger Victor Moses or Colombia wide man Juan Cuadrado.
Chelsea also bought Spain forward Pedro from Barcelona earlier this week and signed goalkeeper Asmir Begovic and striker Radamel Falcao in the close-season.
Jose Mourinho's team, who have taken one point from their opening two league fixtures, visit West Bromwich Albion on Sunday.
(Reporting by Tom Hayward, editing by Tony Jimenez)
