The regulator voted 5-0 to turn down the deal, following a 4-1 vote to reject a request seeking to address concerns voiced by the regulator, the company said.
PNM said it will review and evaluate the order.
Five federal agencies and the Public Utilities Commission of Texas had approved the proposed merger, leaving the New Mexico regulator is the only remaining approval required to complete the deal, according to the company's statement.
Shares of the PNM Resources fell more than 3% in recent Thursday premarket activity.
Price: 44, Change: -1.63, Percent Change: -3.57