The new edition will aim to combine the Huffington Post's "mix of news, blogging, community, and social engagement with El País's legendary journalistic traditions", an announcement by Huffington Post founder Arianna Huffington said.
She added that El Huffington Post "will be rooted in Spanish culture" and "run by Spanish journalists".
"El Huffington Post will be both a journalistic entity and a platform that captures the unique perspectives, voices and spirit of the Spanish people.
"One reason we are particularly thrilled to partner with PRISA and El País is that they are not only deeply familiar with that spirit, but a core part of it, with their long history of championing democracy and free expression.
"Together, we'll be telling the stories people care about most - and, just as important, helping them tell their stories themselves."
According to a news story on the El Pais website El Huffington Post launched at midnight, with the newsroom based "in the same building as El Pais".
El Pais's parent company PRISA is also said to own 50 percent of El Huffington Post.
In October last year the Huffington Post teamed up with French publishers Le Monde and Les Nouvelles Editions Independantes to launch Le Huffington Post.