The 78-year-old Chanel designer described the pleated muslin and organza dress with a ruffled collar as "perfect" for the pregnant bride.
"I love her. She's great," said Lagerfeld Wednesday at the opening of an exhibit celebrating Chanel's signature Little Black Jacket that was curated by Lagerfeld and former French Vogue editor Carine Roitfeld.
Barrymore, 37, married art consultant Will Kopelman on June 2 at her Montecito, Calif., home.
Lagerfeld says he's known Kopelman, the son of former Chanel CEO Arie Kopelman, since he was a child.
He said he and Barrymore tried several options before settling on the custom gown featuring feathers, embroidered flowers and a black satin belt.
His advice for dressing up with a baby bump?
"Be comfortable, even in the wedding dress," he said.
Some of Lagerfeld's other celebrity friends, including Linda Evangelista, joined him at the SoHo gallery event that also launched his new book. "The Little Black Jacket: Chanel's Classic Revisited by Karl Lagerfeld and Carine Roitfeld" features men, women and children wearing the classic piece in creative ways, seeing the jacket transformed into a head wrap or miniskirt.
For one photo, Kanye West poses casually in a version of the jacket with a T-shirt and jeans, while in another Sarah Jessica Parker wears a crown and lets the jacket playfully billow behind her head.
Nicole Evatt covers entertainment for The Associated Press. Follow her on Twitter at http://twitter.com/NicoleEvattSuggest a correction