Background Religion has rarely been studied as a determinant of infant feeding practices. We examined whether religious affiliation is associated with formula feeding vs breastfeeding intention and practice in women from the United States Project Viva cohort. Methods Between 1999 and 2002, 2128 pregnant women were recruited in the area of Boston, Massachusetts. They reported by questionnaire their religious affiliation, and their intended and practiced infant feeding mode (exclusive formula feeding vs partial vs exclusive breastfeeding) at different time points. We examined associations of religious affiliation with infant feeding intention and practice by modified Poisson regression and multinomial logistic regression adjusted for known sociodemographic confounders. Results Of 1637 women with complete data, 52% reported being Catholic, 29% Protestant, 11% unaffiliated, 4% Jewish, and 4% other religion. Overall, 8.5% and 15.9% women intended and initiated exclusive formula feeding, respectively. Compared with unaffiliated women, Catholics were more at risk to intend to exclusively formula-feed their infant at birth (risk ratio [95% CI]: 6.4 [1.6-26.0]) and to exclusively formula-feed after delivery (2.4 [1.3-4.2]) and 3 months postpartum (1.3 [0.98-1.8]). The odds ratio for intending and practicing partial (vs exclusive) breastfeeding did not differ by religious affiliation at most examined time points. Associations of Protestant women with infant feeding exhibited estimates closer to unaffiliated than to Catholic women. Conclusions Catholic women are more at risk to intend and practice exclusive formula feeding than women of other religious affiliations. Our findings may help health care practitioners adapt their breastfeeding promotion to the mother's religious affiliation.