The "Sex and the City" star will replace Demi Moore, who bowed out of the Millennium Films production after being hospitalized earlier this week and subsequently seeking treatment for what her publicist characterized as "exhaustion."
(Moore split from her husband of six years, Ashton Kutcher, late last year amid media reports that he had been cheating on her.)
The role of Steinem, a journalist who went undercover at the Playboy club, will be a cameo.
Also in the film: Chloe Sevigny, who will play Rebecca, a feminist journalist on assignment from a men's magazine to write a story about XXX actress Linda Lovelace.
Amanda Seyfried is playing the title role of Lovelace, who rose to fame in the breakthrough porn film "Deep Throat" but later condemned the industry, claiming that she had been coerced into performing.
"Lovelace" is being directed by Rob Epstein and Jeffrey Friedman, whose credits include "Howl" and "The Celluloid Closet."