U.S. District Court Judge Clark Waddoups' ruling took issue with the phrase "or cohabits with another person," writing that it was "a violation of the Free Exercise Clause of the First Amendment to the United States Constitution and is without rational basis under the Due Process Clause of the Fourteenth Amendment."
His ruling is similar to one from last December, which the judge later reversed. Waddoups' ruling differs because he preserved the language "marry" and "purports to marry" -- meaning adults may not have more than one active marriage license.
The high-profile case was filed by the Brown family, stars of TLC's Sister Wives, who fled the state fearing prosecution.
The Utah Attorney General's office said it was considering appealing the ruling.
Kody Brown and his wives said they hoped Attorney General Sean Reyes would allow the ruling to stand.
"The decision brings closure for our family and further reaffirms the right of all families to be free from government abuse," the Browns said in a statement. "While we know that many people do not approve of plural families, it is our family and based on our religious beliefs."
"Just as we respect the personal and religious choices of other families, we hope that in time all of our neighbors and fellow citizens will come to respect our own choices as part of this wonderful country of different faiths and beliefs," they said.