The 21-year-old midfielder, named the Addicks’ player of the year last season, has committed his future to the club until 2019.
“It’s a good feeling to secure my future,” Cousins, who has been at The Valley since under-13 level, told the club website.
“It’s been talked about all summer and I’m pleased we could get everything sorted before the new season starts.
“I’ve been here for a very long time and I’m proud to come from this academy. It’s my home, I feel part of the club and I want that to continue.
“There’s a really good feeling about the place at the moment. We’ve added some good players to the team and it’s clear that the club want to build towards something positive.
“I’m just really excited to get the season started now.”