Fighters linked to al Qaeda abducted a Spanish journalist and a photographer in a rebel-held eastern Syrian city in September.
Journalist Javier Espinosa and photographer Ricardo Garcia were taken by rebel group the Islamic State in Iraq and the Levant (ISIL) on September 16 at a checkpoint in Raqqa, which fell to militant Islamist brigades in March, the paper reported.
Espinosa, a journalist for the newspaper, and freelance photographer Garcia were just a few kilometers from the border with Turkey and were trying to leave Syria at the time.
The incident had not been reported until now because of negotiations with the rebels holding the men, the newspaper said.
The kidnappers have not said what they want in exchange for the men and El Mundo did not say why it had decided to announce the kidnappings now.
The pair is veteran journalists who have covered the Syrian conflict since its beginning and have travelled to the region around ten times.