The 27-year-old striker has moved to Craven Cottage for an undisclosed fee after agreeing a four-year deal.
"I'm very happy with the signing of Ross McCormack who will be an important player for our squad," Magath told Fulham's official website.
"Ross was not only the top scorer in the Championship last season, but also made a lot of assists, so has a proven pedigree at this level which can only be an asset to us.
"I'm glad for our fans that we've signed an experienced and exciting player, a Scotland international, with a great attitude and presence who can be a leader that our young and ambitious squad can learn from."
McCormack scored 29 goals for Leeds last season.