Snowden received asylum in Russia in August.
Greenwald said there's a "sustained attack" on press freedom in the United States, and he criticized the British government for asking the Guardian newspaper to destroy data leaked by Snowden. He called on journalists everywhere to work together to defend their rights.
"The more we band together, the harder it will be for states to attack press freedoms," he said.
The Miami-based Inter American Press Association has about 1,400 member news organizations and promotes press freedoms throughout the Americas. Earlier speakers at the general assembly included Associated Press president and CEO Gary Pruitt and Cuban dissident blogger Yoani Sanchez, who spoke about arrests of independent journalists and bloggers there.
Pruitt spoke about the U.S. Justice Department's seizure of records of thousands of telephone calls to and from AP reporters as part of an investigation to find the source of a story about a foiled attempt to bomb a U.S.-bound airliner, a move that Greenwald also criticized during his remarks.