"My office has not yet lost a lawsuit to the Trump administration and we do not intend to lose this one," Ferguson said in the release on Tuesday.
The states joining the lawsuit include Massachusetts, California, Delaware, Iowa, Illinois, Maryland, Minnesota, New Jersey, New Mexico, New York, North Carolina, Oregon, Pennsylvania, Rhode Island, Vermont, Virginia, the release noted.
The lawsuit alleges the Trump administration violated the constitutional due process rights of the immigrant parents and their children by separating them without finding that the parents pose a threat to the child, the release said.
The Trump administration came under intense criticism for its "zero-tolerance" immigration policy after images emerged of children locked up in cages in a Texas detention center.
As a result of the policy, nearly 2,000 immigrant children were forcibly separated from their parents between April 19 and May 31 following arrests in connection with illegal entry into the United States, according to the Department of Homeland Security.