The 28-year-old rejoins the Addicks six years after leaving for Huddersfield Town, bringing valuable Premier League and Championship experience.
Grant progressed through Charlton’s academy after joining at the age of 12 and made 98 senior appearances, scoring 21 goals before his 2019 departure.
He later impressed at Huddersfield, earning a move to West Bromwich Albion, where he scored 35 goals in 173 appearances across all competitions.
"It’s an unbelievable feeling. I was a boy when I left and I think I’m coming back as a man,” Grant told club media.
“I’ve got a lot of experience under my belt and I know what I can bring. When I was here before, I was younger and I was raw - I’ve matured a lot as a player and I can’t wait to get back out there.
"I’ve always looked out for Charlton since I left. I’ve been so fond of the club. To be here now, knowing how much Charlton gave to me – (it feels) truly amazing."