The fire broke out around 1:20 a.m. near Chino and Euclid avenues, according to the Ontario Fire Department.
The blaze is several hundred feet long and wide, fire officials said.
Santa Ana winds up to 30 mph were blowing embers, making it difficult for fire crews to battle the blaze, officials said. Smoke can be seen streaming from the agricultural area across neighborhoods in adjacent Chino.
There are no reports of injuries or structures threatened.
The cause of the fire is under investigation.