Santa Ana police said the 13-year-old was visiting for a family event that was scheduled for Sunday.
Authorities were alerted soon after the incident occurred Saturday afternoon, and the boy was taken into custody at a home on the 500 block of N. Porter Street.
"Our officers interviewed the 13-year-old boy who eventually admitted that he had some type of sexual contact with the 8-year-old victim," said Cpl. Anthony Bertagna of the Santa Ana Police Department. "The officers booked the 13-year-old into juvenile hall for these sex charges."
The girl's mother reportedly saw the boy come out of her daughter's bedroom. After examining the girl, the mother took her to the hospital.
The boy was booked for lewd and lascivious acts with a child under 14.