Initial reports for The Seattle Times showed that 56 percent of voters approved of Initiative 502. Most of the support was from the urban areas of the state.
Those who campaigned for the initiative said the tax revenue would bring in at least hundreds of millions of dollars for the state.
Buyers could purchase one ounce of dried marijuana, one pound of marijuana-infused product, such as cookie, or 72 ounces of marijuana-infused liquids.
Colorado also had a ballot measure to legalize marijuana, and early polls showed a closer contest there.
Neither state measure addresses the federal ban on marijuana, setting up a potential court fight.
The measure also establishes a standard blood test limit for driving under the influence.