Ventura County and Los Angeles County firefighters responded to the collision along Highway 126 near Sharp Avenue.
One person was pronounced dead at the scene, and three others were in critical condition, officials said.
Westbound and eastbound lanes were temporarily closed as an investigation into the crash continued.
All lanes were later reopened, and California Highway Patrol officials said the rain likely contributed to the fatal crash.
Preliminary info regarding the fatal collision on SR126 late this afternoon. Adverse weather conditions likely contributed to this fatal collision. We are expecting heavy rain throughout the night and into the morning commute. Please plan more time into your morning commute. pic.twitter.com/PTmut7Wa0v— CHP Moorpark (@CHPMoorpark) January 9, 2018