Outside Terminals:
Yes, there are Smoking Areas at the one end of Terminals at SEATAC, you might need to walk up a bit to get there.
If you have landed at Seattle Tacoma (SEATAC) to catch a connecting flight you may walk out of the terminal to designated smoking area that’s right next to the International Arrival exit, you will need to go through security check again when you enter.
Inside Terminals: Smoking is Prohibited inside terminal buildings, no smoking areas inside terminals.