Rocco Ritchie's Weed Arrest Surfaces

Madonna's son, Rocco, has been arrested in London for possession of weed ... but the arrest flew under the radar for nearly 2 months.

TMZ has confirmed, London police arrested the 16-year-old for possession of cannabis on Sept. 28th. He reportedly incurred the wrath of neighbors who were bothered by what they believed was drug use in their upscale neighborhood when the boy lives with his dad, Guy Ritchie.

According to reports, cops were called and saw Rocco and a friend sitting on a wall with a friend. They claim he tossed something in the air, but they found a small quantity of weed in his backpack and traces on his clothing.

Rocco moved away from his mom almost a year ago to live with his dad in London. She objected, claiming Guy was too permissive and it triggered a nasty court battle. He was seen back in April hanging with friends, and smoking what appeared to be a joint.

Rocco's already been sentenced. He attended a drug ed class.