The 19-year-old moved to Stamford Bridge for £13m in July 2023 from Santos and spent last season on loan at Chelsea’s sister club Strasbourg, making 24 appearances.
In October 2020, aged 15 years and 308 days, he became the youngest player in the history of the Brazilian top flight when he made his Santos debut against Fluminense in the Maracana Stadium.
Chelsea have a sell-on clause for the player, who will link up with Cristiano Ronaldo and Sadio Mane at Al-Nassr.
Elsewhere, Daniel Podence has moved to Al-Shabab from Wolverhampton Wanderers for £5m.
The Portugal winger, 28, has played for Wolves since 2019, making 108 appearances and scoring 16 goals.
He was on loan to his former club Olympiacos last season.
The Saudi transfer window closed on Monday evening.