Lufthansa flight No. 459 left San Francisco around 9 p.m. and, after a few hours, crews got information that there was an electrical smell on the aircraft, Lufthansa spokesman Nils Haupt said.
The captain decided to land at Seattle-Tacoma International Airport and the descent was declared an emergency landing.
The plane landed safely in Seattle just before midnight, Haupt said.
Upon landing, three cabin crewmembers and one passenger were treated aboard the aircraft after reporting feeling dizzy, Haupt said.
There were 323 passengers aboard the plane.
Immediately after landing, technicians boarded the plane to look into the cause of the smell.
Most passengers were booked on other flights or otherwise provided hotel accommodations.
Haupt said an investigation into the cause of the electrical smell is ongoing.
Once the issue has been resolved, the aircraft, an Airbus A340-600, will either return to SFO empty or will fly to Frankfurt from Seattle, he said.