Executive Editor David Boardman says the paper simply can't afford to keep giving away full access to its website.
In a column Sunday, he noted that since The New York Times put up a paywall two years ago, more than 400 daily newspapers across the country have followed suit.
The digital subscription will be free for readers who already have The Seattle Times delivered at least one day a week, and the paper is offering new subscribers an introductory offer of 99 cents per week for Sunday delivery and unlimited online access. Online-only subscriptions will also be available.
People who don't subscribe at all will still have some access to www.seattletimes.com, but if they visit too many times, they'll be required to pay.
Online access to KOMO News will continue to be free.
"While we respect the decision made by the Seattle Times, KOMOnews.com will continue to be completely free for our readers," said Jennifer Kuglin, general manager of Fisher Interactive Network. "We have no plans for switching to a subscription model."