The versatile 19-year-old, who can play in midfield or defence, has made an immediate impact for Chris Powell's side this season and impressed in the 1-1 draw with Nottingham Forest in midweek.
Powell believes the youngster, who first joined the Addicks as a 13-year-old before progressing through their Academy to the first-team, has a bright future ahead of him.
He told the club's official website: "Jordan has made an impact on everyone over the past few weeks and I'm really pleased that he sees his future with Charlton.
"Ever since I have been at the club he is a player I have known about, and known how talented he is, but his performances on our pre-season tour this summer told me he was ready to play a part with the first team.
"It is still very early in his career but he has good attributes and can also play in defence, even though we see him as a midfielder primarily.
"What I also really like about him is that he has the right attitude and a terrific temperament. Nothing seems to faze him and he has a real hunger to succeed."
Cousins added: "I'm delighted. Making my debut this year was a dream come true and now I've signed a new deal it can't get any better for me.
"Playing in the first team is what I've dreamed of doing all my life and I know there's still a long way to go but as of now I'm in a great place.
"It's been a hell of an experience stepping up from U21 level and being in and around the first team, but it's been an experience I've really enjoyed and I feel like I've grown as a person and as a player being in and around so many experienced pros."