The crash happened just after 5:30 am on Highway 180 in Central Fresno as it merges into Downtown.
The California Highway Patrol says a black Toyota Tacoma was going west on Highway 180 when the driver lost control of the truck and went over the guardrail.
The truck landed below on Highway 41.
The passenger was taken to the hospital, where he died.
The driver was also injured but is expected to recover.
Officers say the rainy weather was a factor in the crash.
The slippery road conditions were an issue for this driver and several others across the Valley.
The CHP recommends that all drivers check their tire pressure, adding that having it at the right level will prevent or decrease the odds of hydroplaning.