The "Dawson's Creek" star will appear in the company's advertising campaigns beginning in February, Women's Wear Daily reported Friday.
Holmes said her favorite products from the line -- the caviar anti-aging moisturizing shampoo, conditioner and bamboo dry shampoo -- have "transformed" her hair, People magazine reported.
"I love that Alterna uses natural and organic ingredients and stays away from harsh chemicals like sulfates and parabens," Holmes said Thursday.
This is Holmes' second foray into representing beauty products. She became a spokeswoman for Bobbi Brown Cosmetics in September, E! reported.