Merkel was on a list of some 122 heads of state and government about whom the NSA gathered intelligence in May 2009, the German weekly reported on its website.
Der Spiegel said it had seen documents showing the more than 300 reports on Merkel along with the presidents of Peru, Somalia and Belarus.
The purported NSA files could "be an important element of proof" for the German judiciary, which is to "decide in the coming days whether to open an inquiry into suspected espionage," it said.
Documents made public by Snowden in October revealed that the NSA had listened to Merkel's cellphone conversations for several years — a revelation which strained relations between the two strong allies.
A trove of disclosures by Snowden, now a fugitive granted temporary exile in Russia, resulted in widespread condemnation of spying efforts in the US and abroad, given the vast capabilities of America's intelligence programmes, which US President Barack Obama is now seeking to reform.
SEE ALSO: 'Bundestag should be searched for bugs'