Civil unions "promote an unacceptable lifestyle, undermine the faith of the Church on holy Matrimony, and cause scandal and confusion," said Bishop Thomas Tobin of Providence, R.I.
Tobin reminded Catholics they may not participate in such ceremonies.
"To do so is a very grave violation of the moral law and, thus, seriously sinful," he said in a statement June 30.
The legislation will give same-sex couples who enter into civil unions the same rights and benefits as marriage in Rhode Island.
Some proponents of same-sex marriage said the bill did not go far enough and allowed overly broad religious exemptions.
However, Tobin was "deeply disappointed" at the decision.
"The concept of civil unions is a social experiment that promotes an immoral lifestyle, is a mockery of the institution of marriage as designed by God, undermines the well-being of our families, and poses a threat to religious liberty," he said.