Hundreds gathered at Seattle Center for a New Year's Eve style countdown to 12 a.m., when the legalization measure passed by voters last month took effect.
Under the new law people anyone in Washington who is 21 years or older can have up to one ounce of marijuana for personal use in a private place, such as a home but the catch is there's no legal way to obtain it.
It's a felony to sell it or grow it in Washington, unless it's authorized under the state's medical marijuana law.
According to the King County prosecutor, if police think a driver is under the influence of marijuana, a blood test can be done. The legal limit is five nanograms of THC per millimeter of blood.
The state attorney general also reminded Washington residents that marijuana remains a controlled substance under federal law.