The Cottagers pipped Barclays Premier League rivals Birmingham to the signing of the 23-year-old on a three-year contract from Dutch side AZ Alkmaar.
Dembele, who can also play on the wing, is believed to have cost Fulham £5m.
"I'm glad that we have finalised the deal with Moussa and I'd like to welcome him to the club," Hughes told the club's website.
"Moussa is strong and quick and will add another dimension to our attacking options.
"It's important that we go into the season with some additions to the squad that will enable us to compete at a consistently high level throughout the season.
"I look forward to working with Moussa and, hopefully, to making a few more additions before the transfer window closes."