The occasion was celebrated by dozens of users near Seattle's famed Space Needle tower amid blaring reggae music.
The public gathering defied a key provision of the state's landmark marijuana law, which forbids users from lighting up outside the privacy of their homes.
Hours earlier, Seattle's city attorney issued a stern warning that pot smoking in public would not be tolerated and that violators of the law faced $100 fines.
But the local prosecutor's statement was contradicted by the Seattle Police Department's own instructions to its officers to limit their enforcement actions to warnings.
Passed by voters last month as a ballot measure called Initiative 502, the new marijuana law removes criminal sanctions for anyone 21 or older possessing an ounce or less of pot for personal recreational use. It also legalises possession of up to 16 ounces of solid cannabis-infused goods, such as brownies or cookies.
However, driving under the influence of cannabis, or imbibing in public places remain illegal.