The rock – said to be “the size of an apartment” – fell from El Capitan in Yosemite National Park at the height of climbing season.
At least 30 climbers were on the wall at the time, but it was not clear if the victims were climbers or tourists, park ranger Scott Gediman said.
“It was witnessed by a lot of people,” he said.
The injured person was taken to a hospital near the park.
El Capitan is one of the world’s largest granite monoliths towering 4,000 feet above Yosemite Valley in California.
Several people made emergency calls, reporting the rock fall from the Waterfall route on the east buttress of El Capitan.
Officials didn’t provide details on the size of the rock fall, but climbers posted pictures on social media from hundreds of feet up the wall showing billowing white dust moments after the crash.
“I saw a piece of rock, white granite the size of an apartment building, at least 100 feet by 100 feet, suddenly just come peeling off the wall with no warning,” said Canadian climber Peter Zabrok, 57, who was scaling El Capitan and was above the rock fall.
Zabrok said he has climbed El Capitan dozens of times and has “never seen anything like this.”
Mountaineers from around the world travel to the park in the Sierra Nevada to scale El Capitan’s sheer face. Autumn is one of the peak seasons because the days are long and the weather is warm.