Seattle today becomes the 15th Aer Lingus transatlantic route from Dublin.
Four flights a week will be operated by an Airbus A330-200 aircraft
Connecting fares from 14 UK airports via Dublin to Seattle start at £239 each-way.
Seattle is the second new transatlantic route to be introduced by the Irish flag carrier for this summer, following four flights a week to Philadelphia started in March.
Frequency on the route has been increased from today to offer daily flights during the summer peak due to demand.
The start of the Seattle link also marks the beginning of a partnership with Alaska Airlines.
The interline deal means Aer Lingus travellers can connect onwards via Seattle to destinations along the US west coast, Alaska and Hawaii.
Aer Lingus chief operating officer Mike Rutter, said: “Seattle as a destination holds great promise for Aer Lingus given the strong business ties between the two regions, making this an important route for business travel as well as leisure trips as exemplified by the high demand for business class tickets on the route to date.
“Our interline agreement with Alaska Airlines means we can offer seamless onwards connections to our guests to 40 additional locations serviced by Alaskan Airlines.
“It also ensures that we can make Ireland and further into Europe accessible for Alaskan’s 44 million travellers.”