No, Seattle's all-time record low was 0 degrees set on Jan. 31 in 1950. Really, Seattle only has a handful of record lows in single digits -- mainly from the ultra-harsh winters of 1950 and 1951 here. But Seattle rarely drops below 20 degrees anymore.
We can thank the relatively warm waters of the Pacific Ocean for keeping our winter temperatures from getting too cold. Also, as the city grew, it now creates it's own heat at night (known as the urban heat effect) that also keeps the nights warmer.