Speaking to a press association, he said he would report about each case in the region and warned that more spying within the United States would also be revealed.
Greenwald's comments came as France and Mexico angrily demanded swift explanations Monday about fresh leaks by former US security contractor Edward Snowden.
The leaks alleged the United States had spied on millions of French phone communications and that the National Security Agency (NSA) had also hacked into former Mexican president Felipe Calderon's email account.
Greenwald, an American journalist living in Brazil, spoke via video conference to the 69th assembly of the Inter American Press Association (IAPA).
Multiple Latin American meetings were monitored, Greenwald said, including those of the Organization of American States (OAS), as well as talks on free trade treaties, although he did not go into more details.
According to Greenwald, disclosures released Monday by the French newspaper Le Monde, which created controversy between Paris and Washington, had been in the hands of the French daily for some time.
The allegations, the latest from leaks by Snowden, marred a visit to Paris by US Secretary of State John Kerry, where he discussed moves to try to end the war in Syria.
The reporter, who resigned last week from British daily The Guardian, told the assembly that documents being leaked by Snowden are kept in different parts of the world.
Greenwald, a lawyer by training, has said he will now devote his energy to the launch of a new journalism project backed by eBay founder Pierre Omidyar.
The annual IAPA meeting at which Greenwald spoke brought together some 300 journalists from the Americas, who arrived in Denver, Colorado, on Friday and will stay until Tuesday.