It said the alleged spying violated Spain's data protection law and that the newspaper had called on the prosecutor's office last week to open an investigation.
The newspaper's chief news editor, Vicente Lozano Garcia, said the company had no problem handing over the documents given that a judicial inquiry had been opened and that El Mundo had already published them.
El Mundo said the documents were among those leaked by former NSA contractor Edward Snowden, who is wanted by the United States but has been granted asylum in Russia.
El Mundo's stories last week were co-written by Glenn Greenwald, a journalist who originally revealed the NSA surveillance program based on leaks from Snowden. El Mundo said it had reached a deal with Greenwald to have the exclusive on the Snowden documents relating to Spain.
Allegations of NSA spying on allies have irked European countries. Spain insists it is unaware of any spying but demands an explanation from Washington. Spain's intelligence services chief will brief Parliament behind closed doors on Wednesday.
The NSA acknowledges European phones were monitored as part of a NATO program but insists it didn't act alone.