There was reported Premier League interested in Solly, and manager Chris Powell was delighted to have wrapped up the signature of the player.
"I'm really pleased we've managed to get Chris done," Powell told the club's official website. "It's fully deserved after his exploits over the last couple of seasons since I've been here.
"It's important to get your players, especially young players that have made an impression, tied down to the club and he's an academy product that has blossomed in the side and has gone from strength to strength.
"It's always important and it's a good sign that we've been able to do it and I'm really pleased for Chris because now he can fully concentrate on his football."
The 22-year-old has played 114 times in the league for Charlton.