The five-times-weekly operation is the first year-round service between the destinations, with Delta Air Lines operating a summer-only schedule.
Portland will become BA’s 27th US destination and will take the number of transatlantic flights with joint venture partner American Airlines to 86 per day.
Flights will depart at 15.05 on Mondays, Wednesdays, Fridays, Saturdays and Sundays, arriving in Portland at 16.55.
Return flights will leave Portland at 18.45 and arrive at Heathrow at 12.10 the following day.
Services will be operated by B787 Dreamliner aircraft and will have a journey time of around 10 hours and 20 minutes.
“Portland will be a fantastic addition to our route map,” says Neil Chernoff, British Airways’ Director of Networks and Alliances.
“As a major technology and innovation hub the city is drawing ever-increasing numbers of international visitors.”
Chernoff adds: “Since 2015 we’ve launched routes to Pittsburgh, Charleston, Nashville, New Orleans and San Jose, and this is key to our joint business strategy which is growing across North America.
“We’re confident the route will be warmly welcomed by both business and leisure travellers.”
Return fares will start from £459 in World Traveller, £758 in World Traveller Plus and £1,399 in Club World.