The Addicks are set beat off competition from Birmingham, Cardiff, Derby, Lincoln and Oxford to sign the 19-year-old. Palace will also see him renew his contract until 2025.
The move represents a coup for Charlton after his stunning 19 goal season in PL2 last year.
Rak-Sakyi, who made his Premier League debut last term, completed his medical on Thursday morning.
Charlton beat Championship side QPR on penalties in midweek Carabao Cup action and manager Ben Garner is confident of extending their strong start to the season away to Sheffield Wednesday.
“I think we have to go with full respect because they’re a very good team,” said the Addicks boss. “I know Darren Moore well, he’s a great guy. He’s an absolute gentleman in the football world.
“We go there with respect and complete understanding of what they do but also with the attitude that we’re going there to win.
“We want to play the way we want to play. We know it’s a tough place but we want to embrace those challenges and look forward to them. After tonight we can certainly look forward to it.”