Gibbons said in a statement released during the Memorial Day holiday that Senate Bill 283 conflicts with a state constitutional amendment approved by Nevada voters in 2002 supporting marriage between a man and a woman.
"Only the voters should have the right to undo or amend constitutional mandates," he said.
It was not immediately clear if the Legislature would seek to override Gibbons' veto.
The measure would prohibit discrimination against domestic partners, and give them the same community property and debt rights as married couples.
The Republican governor also said domestic partners can sign private contracts to accomplish many of the goals of the law.
Harrah's Entertainment has called for an override if Gibbons vetoed the measure.
The casino giant pointed to an estimated $700 billion in buying power among potential lesbian, gay, bisexual and transgender customers.
The bill also had support from of the powerful Nevada Resort Association, and drew praise from the American Civil Liberties Union of Nevada.
Gary Peck, ACLU executive in Las Vegas, called domestic partnership rights an economic and civil rights issue.
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