With the park as a backdrop, Salazar also said his agency would complete an environmental review of a plan to withdraw the full one-million acres from new mining activities for the next 20 years.
The temporary moratorium was set to expire next month.
Interior officials were not prepared to issue a final decision on the 20-year withdrawal, Salazar said in extending the temporary ban through December 20.
"Wisdom, caution and science should guide our protection of the Grand Canyon," the secretary said in a news conference at the park's Mather Point amphitheater. "In this moment we face a choice that could profoundly affect the Grand Canyon in ways we do not yet understand."
In 2009, the Obama administration imposed a two-year ban on uranium mining on federal lands around the park so authorities could review environmental and economic impacts while developing a long-term policy.
Since then, environmentalists, Native American groups, recreationalists and others have pressured Salazar to ban mines on an estimated 0ne-million acres, arguing that the industry would contaminate water and spoil one of the planet's natural wonders.
Salazar said his decision to extend the temporary ban was based on concerns about water quality at the canyon as well as the economic effects of allowing mining companies to stake new claims.
Jon Jarvis, director of the National Park Service, supported Salazar's decision, noting the uncertainties that remain about allowing additional uranium-mining claims.
"From our perspective, when there are significant unknowns, you err on the side of conservation," Jarvis said, "so that managers down the road will be in a position to make better decisions. There is really only one Grand Canyon."
Mining opponents recently took out an ad in the New York Times -- an open letter to the president saying the Grand Canyon is a "national treasure" under threat.
Rep. Raúl Grijalva, D-Arizona, who has introduced a bill to permanently protect the lands, cheered Monday's decision.
"The Grand Canyon is one of the most recognizable and spectacular natural wonders in the entire world," he said. "This is the right decision for today, for tomorrow and for the future of our country."
Grijalva attended the news conference, along with representatives from Indian tribes and other communities.
The secretary was accompanied by Jarvis, Bob Abbey, director of the Bureau of Land Management; and Marcia McNutt, director of the U.S. Geological Survey.
All those agencies would be affected by a decision on the uranium issue.