Democratic Gov. Ronnie Musgrove signed the bill after it easily cleared the Legislature in the final days of the recently concluded 2000 session. Florida and Utah have similar laws.
The American Civil Liberties Union has vowed to file a lawsuit on behalf of an unidentified gay couple planning an adoption in Mississippi. The ACLU is already fighting Florida's law in court.
Mississippi's law takes effect July 1. Its supporters said it was spurred in part by Vermont's new law giving gay couples nearly all of the benefits of marriage.
"We need to put up a firewall and say, 'This is not going to happen here,'" Mike Crook, state director for the Tupelo-based American Family Association, said Wednesday. "They can go to court all day long, and I think we'll prevail."
Opponents said there was no reason for the state to get involved in the issue.
"It's a part of the get 'em mentality. They have to be getting somebody," said Democratic state Rep. Jim Evans.
The ban had divided religious groups. The state's top Episcopal leader urged its defeat. Baptists and Methodists lobbied for it.
Connecticut took a step in the opposite direction Wednesday, as the state's Senate gave final legislative approval to a bill that would allow gays and other unmarried people to adopt their partners' children. Gov. John G. Rowland has not said whether he will sign the bill.
Late last month, Vermont recognized civil unions between gays and lesbians, creating a parallel to marriage that includes all 300 state rights heterosexual couples have.
Thirty states have enacted laws refusing to recognize gay marriages performed in other states.
By GINA HOLLAND