A Los Angeles County sheriff's official said 19-year-old Michael Sykes was arrested Monday morning at his home in Moreno Valley.
Police say Sykes and at least six others sexually assaulted the girl on March 10 at about 9 p.m. in a bathroom at Victoriano Park in the 25700 block of Los Cabos Drive. The park is next to an elementary school and across the street from several homes.
"It's horrible, absolutely horrible," said Evan Everett of Moreno Valley.
An older girl who knew the victim was also arrested. Authorities have said they believe she lured the girl into the bathroom so the boys could attack her.
No other details were released about the assault, and the condition of the young victim is not yet known.
The six other male suspects involved in the attack have already been arrested, but since they're under age, they were booked into juvenile hall. Police say all of the suspects are believed to be associated with a local street gang.
Police said news of the gang rape was not released to the public earlier because they did not want to jeopardize the case.
Anyone with information is urged to call the Moreno Valley Police Department at (951) 247-8700.
The Associated Press contributed to this report.