The race course started and ended near the Experience Music Project and took runners along some of the prettiest stretches of the city, which could be seen through a thick fog.
Thousands of runners braved the cold to raise money for important causes.
Trisha Westly came all the way from the East Coast.
"I trained for New York under Autism Speaks, raised $3,000 and - as you know - the hurricane cancelled the race," says Westly.
Another runner was raising funds to treat a 5-year-old girl with cancer who already has undergone two years of chemotherapy.
Despite the cold and fog, the racers were in high spirits.
"It's a little chilly out here, but it's good it's good weather for racing," said runner Whitney Gibson.
Even though it was dry, some still wore plastic trash bags.
"Keeps us warm while we're waiting for the run," said one.
More than 15,000 runners hit the streets for annual Seattle Marathon - and these athletes say you don't just show up to take on the hilly course.
"Lots of training. I'm always walking and running," said one man.
Competitors in Sunday's race could qualify for the famous Boston Marathon. Others just wanted to know if they could complete the endurance test.
"They are getting ready to get married, so this is how they are bonding," one runner confided about his racing friends.
A grueling run, not for the faint of heart.