A tip to the FBI led investigators to Yukun Jia, said Bronwyn Hogan, spokeswoman for the San Mateo County sheriff's office.
"She is safe with relatives, and in good spirits," Hogan said.
Hogan and FBI spokesman Andrew Black declined to name the city where the girl was staying.
There was no immediate word on how she got cross-country.
Jia flew from Beijing and cleared customs around 2:10 p.m. Thursday. She was reported missing around 5 p.m. by the adult leaders of her group.
The group was headed to the U.S. Space & Rocket Center's space camp in Huntsville, Ala. Center spokesman Al Whitaker said the camp frequently hosts visitors from around the world.
After arriving at the airport, the group toured nearby Stanford University. When adults counted heads before the group boarded the bus, they noticed Jia was missing, police Sgt. Larry Ratti said.
It was not immediately clear whether she disappeared at the airport or at Stanford, but tour leaders told police that Jia's luggage was not loaded onto the bus at the airport. The girl had no known history of running away and no known relatives in the area, Ratti said.
The possibility that the girl was seeking to defect from China hadn't been ruled out, Ratti said. Virginia Kice, a regional spokeswoman for the Immigration and Naturalization Service, said it would have been highly unusual for a child so young to defect. Ratti said the girl speaks Mandarin Chinese, and very little English.
The California Highway Patrol had issued a statewide "Amber Alert," which helped authorities rescue two missing teenage girls Thursday. Ratti said an alert typically is not issued unless authorities confirm an abduction, but authorities offered to distribute Jia's description anyway.
"The CHP said if you've got that info, why don't we put it out there," Ratti said.