The two actresses join previous celebrities including Zooey Deschanel and Kate Bosworth for the campaign, which shoots commercials that take a look at the celebrities and their style while singing the brand's jingle.
Rossum and Belle's adverts will have a Tale of Two Cities theme inspired by their hometowns - New York for Rossum and Los Angeles for Belle - reports Women's Wear Daily.
Rossum's television spot debuts tonight on Fox, ABC and The CW along with other cable networks, while Belle's advert won't be shown for another two weeks.
Rossum's Showtime series Shameless wrapped its second season on April 1.
Next up for the actress are Way Down South and Beautiful Creatures, both out in 2013.
Watch Emmy Rossum's advert below: