The California Highway Patrol says the crash happened just after 4 a.m. Wednesday on northbound Highway 99 near McKinley Avenue.
RELATED: ABC30 Live Traffic Tracker
Officers say they first received a call about a naked man walking on the highway, and minutes later, another call came in saying the man had been struck by a vehicle.
Investigators determined the man had walked directly into the path of a semi-truck traveling north in the No. 3 lane.
The 51-year-old semi-truck driver swerved to avoid the man but struck and killed him.
The man has not been identified.
The driver of the semi-truck stayed on scene and cooperated with officers.
Drugs or alcohol were not a factor for the driver but it is not known if the man was under the influence.
Traffic was backed up to Highway 180 and down to one lane during the crash investigation, but has since reopened.
Stay with ABC30 for the latest details on this developing story.