The 18-year-old has made 11 appearances for the Championship side this season.
Leeds boss Brian McDermott has said that he is pleased to secure Mowatt's services until 2017.
"Everyone at Leeds United is delighted that he has committed his long-term future to us," McDermott told the club's official website.
"Playing regular football as an 18-year-old in the Championship tells you exactly what we think of him."
Mowatt made his Leeds debut against Doncaster Rovers in the League Cup in August.