Bynes, who starred in the films "Hairspray" and "What a Girl Wants," will appear in the ad in December's magazines.
The ad copy reads: "Metamorphosis. What's changed since the 'Amanda Show?' Me. Thanks to milk. Some studies suggest teens who choose milk over sugary drinks tend to be leaner, plus the protein helps build muscle. Grow beautiful, inside and out."
Other celebrities to appear in the campaign are Hayden Panettiere, Beyonce Knowles and the cast of "High School Musical 2."