Police reported to the scene in the southbound lanes of Harbor Blvd., south of Valencia Mesa Dr., shortly after receiving a call from the OCTA bus driver at around 6:38 p.m.
Officers arrived to find the man unresponsive along with the OCTA bus driver that remained on the scene.
The pedestrian was immediately transported to the hospital where he later passed away.
Officers suspect the man was walking in the traffic lanes when he was hit by the bus.
The identity of the victim was not released.
Anyone with additional information is encouraged to contact Fullerton Police Investigator H. Barclay at (714) 738-6815.
Anonymous tips can be sent to Orange County Crime Stoppers at 1(855) TIP-OCCS.