The incident occurred around 11:10 a.m. at the intersection of Florence Avenue and Crenshaw Boulevard.
Video shows a white SUV pull up behind the parked truck. Then, three thieves armed with rifles get out of the car, pin the guard down and take $15,000 in cash.
Witnesses say they didn't even know the robbery had happened until they saw police arrive at the scene.
Authorities are now looking for the suspects. If you have any information on the suspects or recognize them in the video, you are urged to contact the LAPD.