Pol Senior Sgt-Major Kanchit Sridanuch said he was at a checkpoint near Chao Pho Sua Shrine in Bang Khen district when a motorcyclist handed him the bag at 10pm.
The motorcyclist said he had seen the bag fall from a Toyota pickup on Soi Indra 5 so he picked it up.
Kanchit opened the bag and found the drugs inside so he reported the incident to his commander at the Bang Khen police station.
The motorcyclist left the scene without being identified.