The governing Popular Party had appealed against the law, claiming marriage inferred a union between a man and a woman.
But Spain’s Constitutional Court upheld the law, meaning gay marriage will continue to be legal in Spain.
Spain’s Justice Minister Alberto Ruiz Gallardon says the Popular Party will accept the Constitutional Court’s verdict and will not reform the current legislation.
The law on gay marriage will stay as it is for the foreseeable future.
The Spanish parliament passed the gay marriage law when it was under Socialist control in 2005.
The opposing Popular Party announced a revote when they took power late last year after the Socialists were ousted.
The law angered the Roman Catholic Church but polls showed it was backed by most Spaniards.
Since then, more than 22,000 gay marriages have taken place in Spain.