The teenager can operate as both a striker and a winger and, last season, was top scorer for the Gunners' U21 side, netting 17 goals in 30 matches.
Addicks boss Karl Robinson told the clubs official web site: Stephy is someone that I've seen a few times in the U23 Premier League and he has certainly got the key components to succeed in what I call 'real' football. He has the power, the game intelligence and this move is one that he has to grab with both hands.
"They speak highly of him in the Arsenal youth set-up and he will give us something different to Lee Novak, Tony Watt and Josh Magennis. With Nicky going out on loan, we needed to get someone who can help us to stretch a game. Stephy is a real talent and I'm sure Charlton will be a real home for him in the short term."
Mavididi added: "I'm really excited. This is my first loan, it's a big opportunity for me and a stepping stone in my career. It's something I want to do and I'm looking forward to the experience.
"I was at the Millwall game a couple of weeks ago. The Valley is such a big stadium and what captured me the most was the fans and the atmosphere. It made me want to run out in front of them and play. I can't wait to get started."