STORY: Video footage showed a man and a woman gluing themselves to "La Maja Vestida" (The Clothed Maja) and his "La Maja Desnuda" (The Naked Maja) in Madrid's Prado museum.
"+1.5 C" was painted on the wall between the two works.
One of the protesters, who identified himself as "Samuel", said he was protesting because of the "climate crisis" and called on the government to "take action".
Campaign group Futuro Vegetal said its members carried out the protest.
Groups of climate activists have mounted a series of similar protests in recent weeks in the build-up to next week's COP27 climate change conference in Egypt.