The 22-year-old is a product of the Addicks' youth academy and has featured 86 times for the first team, contributing 11 goals.
"I think Callum is a player the fans will love if they like attractive, attacking football," said Reading manager Jaap Stam.
"He is the type of winger who is difficult for opponents to predict – he has a lot of pace, he's good on the ball, very strong in 1v1 situations and he has goalscoring capabilities as well. I'm sure he will be a great asset to the team."
Harriott, who will take the number 15 shirt at the Madjeski, is Reading's eighth signing of the summer window.