American Airlines flight 930 encountered unanticipated moderate turbulence while operating from São Paulo/Guarulhos International Airport (SBGR), São Paulo, Brazil to Miami International Airport (KMIA), Miami, Florida on a scheduled international passenger flight. During preflight preparations, the flight crew noted an area of weather instability north of São Paulo. The captain was the pilot monitoring, and the first officer was the pilot flying with another first officer on rest break. The seat belt sign was illuminated, and the flight crew briefed the cabin crew regarding potential turbulence. At 38,000 feet (FL380), after clearing the previously identified weather instability, the airplane encountered two successive jolts of moderate turbulence lasting about five seconds. The flight attendants (FAs) reported service was concluding and they were not seated at that time. Several FAs and one passenger sustained minor injuries when service items and carts became airborne. One passenger found on the floor near door 3L was seriously injured. After coordinating with the cabin crew and consulting with an onboard physician, the flight crew declared a medical emergency and returned to SBGR. The captain assumed the pilot flying duties and conducted an overweight landing on runway 10L without further incident.