The company said no agreement has yet been made on the stake. Financial newspaper Expansion reported earlier that Repsol was in talks to sell the entire stake to CVC Capital Partners.
"Repsol has received interest from a number of investors, including CVC, with the aim of exploring divestment possibilities for its Gas Natural stake," Repsol said in a statement to the market regulator.
The oil company has long been rumoured to be considering the sale of the non-strategic asset as part of its plan to keep its investment grade via debt reduction.
Europe's fifth-largest refiner by market value is on track to cut its debt to below 7 billion euros by the end of 2017, down from 8.1 billion euros at the end of 2016 and around 12 billion euros a year earlier.
A sale would allow Repsol to speed up the return of a cash dividend and begin investing again after years of focus on debt reduction, analysts said.
Repsol and Criteria Caixa, the holding company that owns Caixabank which in turn owns 24 percent of Repsol, agreed in 2016 to each sell a 10 percent stake in Gas Natural to Global Infrastructure Partners. That deal was worth nearly 4 billion euros in total and took Repsol's stake down to 20 percent.
At 1215 GMT, Repsol shares were up 0.1 percent or 0.02 euros to 15.8 euros per share while Spain's Ibex index was slightly down.