The plane was forced to stop at the airport on the west coast of Ireland at 10:24 GMT on Sunday due to technical problems.
The same jet was diverted to Shannon on Friday night as it headed to Boston from London.
There were 220 passengers on board on Sunday; no one was injured.
A replacement flight left Shannon Airport at 16:55 GMT. BA has apologised to passengers.
A BA spokesman said: "Our flight diverted to Shannon due to a minor technical issue and our cabin crew looked after the customers on board.
"Safety is always our first priority, and the aircraft diverted purely as a precaution."
The flight did not declare an emergency and operations were reported to be normal at all times throughout the incident.
A British Airways plane travelling from Seattle to Heathrow has made its second unscheduled landing at Shannon Airport in three days.