Forward progress of the blaze was stopped at 3 acres near Via Verde around 10:20 a.m., fire officials said.
AIR7 HD was over the scene as crews mounted a ground and air attack, dumping 1,000 gallons of water on the fire.
Traffic was backed up for several miles as the two right lanes of the southbound SR-57 near San Dimas Avenue were shut down. They reopened a few hours later.
It is unclear if any injuries were reported, or if any structures were damaged.
The cause of the fire remains under investigation.