According to UC Berkeley Police, the alleged assault took place overnight Saturday during an overnight summer camp program being held on campus.
Authorities say they were first notified between approximately 1:15 a.m. and 2 a.m. about what was initially reported as an attempted sexual assault inside a dormitory.
Following an investigation, police determined that the incident was not an attempt, but a completed sexual assault.
The suspect, a 27-year-old man from Vallejo who is believed to be a staffer with the camp, was arrested and booked into Santa Rita Jail.
Jail records show he is being held on $425,000 bail.
He is expected to be arraigned on Wednesday.
Editorial Note: ABC7 Eyewitness News is not naming the suspect at this time because he has not yet been formally arraigned
Police say officers provided immediate safety measures and support services for the child.
The incident has shaken members of the campus community, many expressing concern over the safety of children participating in programs held on university grounds.
"Especially for a young kid, that's going to be traumatizing for them," said Francis Wang, a UC employee.
A university spokesperson confirmed that the summer camp involved was not operated by UC Berkeley, though it remains unclear which organization was responsible for running the program.
Residents and community members say the case raises broader concerns about oversight and safety at youth camps.
"I can't believe this is happening," said Berkeley resident Bowen Giang. "I've been to a lot of camps in the U.S. - as a kid, this is definitely terrifying, not only for the child, but for parents and everyone in that environment."
The investigation remains ongoing.
Anyone with information is asked to call UC Berkeley police at 510-642-6760.
If you're on the ABC7 News app, click here to watch live