As of midnight, anyone aged 21 and over could legally carry up to one ounce of cannabis, although smoking it in public remains illegal.
Marijuana use has been legal in the state for medical reasons since 1998.
A law legalizing gay marriage also came into effect in Washington state on Thursday.
Washington, Maine and Maryland became the first U.S. states to back same-sex marriage in a popular vote when they all did so on Election Day. Washington is the first of those to enact the law.
The first same-sex weddings in the state are expected on Sunday.
Gay marriage is permitted in six states, but those laws were passed by legislators or by courts.
But perhaps most notably on Election Day, Washington and Colorado both voted to decriminalize marijuana, which remains illegal under federal law.
However, there will be nowhere to purchase marijuana legally in Washington for at least another year.
It remains unclear how federal law enforcement agencies will deal with liberalization of drug laws in Washington and eventually Colorado.
Any decision to crack down on states with liberal drug laws could affect Washington’s plans to raise tax revenues from a licensed and controlled marijuana market.
“We’re in uncharted water here,” Seattle City Attorney Pete Holmes said at a news conference on Wednesday, according to the Seattle Post-Intelligencer. “We’ve struggled with prohibition for a century. It’s going to take some time to substitute a state licensed and regulated system.”
That means the state Liquor Control Board will have to move quickly to devise first-in-the-nation licenses for marijuana growers, processors and retailers. Until then, the only clearly legal way — at least, under state law — is for a medical marijuana patient to get medicine from a collective garden.
Backers of the Washington law insist it does not encourage or require anyone to break federal law.
But a regional federal prosecutor in the state, Jenny Durkan, told the Post-Intelligencer that “growing, selling or possessing any amount of marijuana remains illegal under federal law,” no matter what state law comes into effect in Washington.
The drug remained in the same category as cocaine, heroin and methamphetamine, she said, adding that only Congress could change that designation.
It remains against federal law to bring any amount of marijuana onto federal property, including all federal buildings, national parks and forests, military installations, and courthouses.
A public celebration of the state’s new marijuana law is planned at Seattle Center, beginning at 7 p.m. on Thursday.