The accident happened at the Baldwin Park Boulevard off-ramp shortly after 4:30 a.m.
The bus veered off the road and crashed into a small drainage canal, hit a light pole and came to lean on a fence. The fence blocked the door of the bus, making it difficult for crews to reach passengers. Crews say they had to force entry into the bus.
There were 39 people on board, including the driver. The driver was not transported to the hospital.
The bus is owned by Travel Tours Americas International out of Hacienda Heights and was returning from the Fantasy Springs Resort Casino in Indio.
The off-ramp was closed for about four hours so the bus could be removed from the ditch, CHP officials said.
The cause of the accident remains under investigation, but authorities said speed may have been a factor.
"We believe the tour bus was traveling 10 westbound, going to exit the Baldwin Park Boulevard. Perhaps the driver was going at an unsafe speed for the off-ramp and that's what caused him to veer off to the right and crash against that fence," said CHP Officer Rodrigo Jimenez.
The accident was the third involving tour buses in just four days. The first incident occurred Thursday afternoon in Fallbrook on the 15 Freeway. A tour bus carrying 22 people flipped on its side. Tayde Murguia, 64, of Van Nuys was killed and 19 others were injured.
Then just minutes later, further up on the 15 Freeway in Corona, another tour bus overturned and hit the center divider. Khang Tieu, 91, of Midway was killed and 23 passengers were injured.
Both buses were heading back from casinos in the rain and the California Highway Patrol said both drivers may have lost control because they were driving too fast on slick roads.