The letter, which also speaks out against same-sex marriage, comes as a bit of a surprise since Brad Pitt and his fiancee, Angelina Jolie, are huge supporters of Democratic President Barack Obama and gay rights.
Jane Pitt's letter was published Tuesday under the headline "Election -- Casting ballot deserves prayerful consideration," E! said.
In her letter, Pitt urged her fellow Christians to "spend much time in prayer before refusing to vote for a family man [Romney] with high morals, business experience, who is against abortion, and shares Christian conviction concerning homosexuality."
She also said anyone who doesn't vote for Romney for president or writes in a name on the ballot during the election is supporting Obama, "a liberal who supports the killing of unborn babies and same-sex marriage."
E! said Brad Pitt's publicist has not responded to requests for comment.