The oak limb fell at about 5 a.m. (8 a.m. ET) and hit the tent in the Upper Pines Campground in Yosemite Valley.
"Park Rangers responded to the campground to provide medical assistance, yet both minors were deceased," the park said.
Authorities did not release details about the young victims except to say they were there with extended family and friends.
"Tree limbs fall all the time for various reasons," ranger Scott Gediman said. "And for two minors to be killed is horrible."
3rd most visited national park
Set on nearly 750,000 acres in Northern California, Yosemite is one of the most popular and beloved sites in the National Park Service system. About 4 million people go there each year, making it America's third most visited national park
These visitors come to enjoy Yosemite's steep rocky facades sporting names like Half Dome and El Capitan, hike through the towering trees that cover much of the grounds, and -- especially in the summer, its busiest season -- enjoy waterfalls and crisp mountain streams.
Like the rest of California, Yosemite has been affected by a historic drought that has dried up streams and creeks as well as parched many trees. Fire restrictions
are in effect for all park areas below 6,000 feet except for certain campgrounds and picnic areas.
'It seems like just a freak accident'
That said, authorities haven't given any indication yet that dry conditions had anything to do with Friday's incident.
"We don't know what caused the limb to fall," Yosemite spokeswoman Jodi Bailey said. "It seems like just a freak accident."
According to Bailey, conditions were clear around Yosemite Valley around 5 a.m. That means no thunderstorm and, thus, no lightning that might have suddenly struck the tree.
There's precedent for this kind of calamity, such as in 2012, when a concessions employee died after a falling limb struck his cabin, Gediman said.
Park officials' focus is taking care of the victims' families as well trying to figure out why the limb fell.
"We are investigating what happened," Bailey said, "but (we are) not sure if we will ever be able to determine exactly what caused it."