The 22-year-old's full-time contract at Elland Road will become effective from the summer, but he is expected to go straight into boss Brian McDermott's squad for Saturday's derby clash at Sheffield Wednesday.
McDermott said in a Leeds statement: "Cameron fits the bill as the type of player we are looking to recruit here. He has pace, he gives us something different and he is hungry to do well."
Stewart has joined Leeds on an initial loan deal due to FIFA regulations preventing a player from featuring for more than two clubs in a season, other than on an emergency loan basis.
McDermott added: "We were aware of Cameron's situation and rules are rules.
"By joining us in this manner, it means we have a player who we know will be here for the next three-and-a-half years and we look forward to working with him for the long-term."
Leeds chairman Salah Nooruddin said: "We're delighted we've been able to secure the signature of Cameron.
"He is a young player who was attracting a lot of interest from clubs in the Championship, including a couple who are above us in the table at the moment, and he is a player Brian was keen to sign."
Stewart, who scored against Leeds in their 4-2 win at Charlton in November, started out at Manchester United and had loan spells at Yeovil and Hull before signing a permanent deal with the latter in January 2011.
He made 55 appearances for Hull and also had loan spells at Burnley and Blackburn before featuring 18 times for Charlton on loan this season.
- Sports & Recreation
- Brian McDermott