Initiative 502 would create a system of state-licensed marijuana growers, processors and retail stores, where adults over 21 can buy up to an ounce. It also establishes a standard blood test limit for driving under the influence.
"Today the state of Washington looked at 70 years of marijuana prohibition and said it's time for a new approach," said Alison Holcomb, campaign manager for the initiative.
With 50 percent of precincts reporting, the measure was passing by a vote of 55 percent to 45 percent.
Supporters celebrated with joints on a sidewalk outside the campaign party in downtown Seattle.
"I've been selling pot for 38 years," said supporter Ben Schroeter. "I've been busted multiple times, most recently eight days ago. Prohibition is stupid. We've known for decades it's stupid and this is extremely important."
Legalization could help bring in hundreds of millions of dollars a year in pot taxes, reduce small-time pot-related arrests and give supporters a chance to show whether decriminalization is a viable strategy in the war on drugs.
A related measure also passed in Colorado, while another marijuana legalization initiative in Oregon was defeated.
The Washington state measure was opposed by Derek Franklin, president of the Washington Association for Substance Abuse and Violence Prevention.
"Legalizing is going to increase marijuana use among kids and really create a mess with the federal government," Franklin said. "It's a bit of a tragedy for the state."
The sales won't start until state officials make rules to govern the legal weed industry, a process that could take a year.
"We have a lot of work ahead," Holcomb said. "The biggest issue I-502 presents for the federal government is that we are creating a robust regulatory scheme."
She hoped the year of rulemaking would provide time to persuade federal authorities that the measure dovetailed with federal interests.
Promoted by New Approach Washington, I-502 calls for a 25 percent excise tax at each stage from the growers on until it is sold in stores to adults 21 and over.
They could buy up to an ounce of dried marijuana; one pound of marijuana-infused product in solid form, such as brownies; or 72 ounces of marijuana-infused liquids.
The cannabis would be subject to testing to establish its THC content, and labeled accordingly.
State financial experts estimate it could raise nearly $2 billion in tax revenue over the next five years, with the money going toward education, health care, substance abuse prevention and basic government services.
When state and federal laws conflict, federal law takes precedence. Federal authorities could sue in an attempt to block I-502 from taking effect. The Justice Department has given no hints about its plans.
The campaign was notable for its sponsors and supporters, who ranged from public health experts to two of the DOJ's top former officials in Seattle, U.S. Attorneys John McKay and Kate Pflaumer.
The effort raised more than $6 million in contributions, with more than $2 million of that coming from Progressive Insurance Co. founder Peter Lewis, who used marijuana to treat pain from a leg amputation.
The ample fundraising allowed New Approach Washington to run television ads through the campaign's final weeks.
Meanwhile, I-502 had little organized opposition. Some in law enforcement and public health are concerned that increased access will lead to increased abuse, especially among teens.
Others who opposed the measure did so because it didn't go far enough, and that the blood test limits were arbitrary and could affect medical marijuana patients. Still others worried about a possible federal-state law clash.
For many voters, it came down to the notion that decades of marijuana prohibition have done more harm than good.
"It's ridiculous to be trying to maintain the law enforcement effort - all the people, all that money, all those resources - to prosecute marijuana use," supporter Karla Oman said. "Tax it, legalize it, everybody wins."
Sean Saulter, 30, of Seattle voted for the initiative because he wanted to see the issue go before the U.S. Supreme Court.
For George Cannon, 43, of Seattle, it was an issue of personal freedom. "I'm not into getting into other people's business," he said.
Initiative 502 found strong support among liberals and moderates, Democrats and those with more than a high school degree. Independents and women were split on the issue, as were suburbanites. I-502 fared well in King County and the Puget Sound area, but not in Eastern Washington, Southwest Washington or on the Olympic Peninsula.
Opposition came from voters 65 and older, conservatives, Republicans and those with a high school degree or less. Weekly churchgoers rejected the measure, while those who said they never attend religious services or considered themselves occasional churchgoers favored legalizing pot.
The initiative led among voters with family incomes of $100,000 or more, but was about even in lower income groups.
The survey of Washington state voters was conducted for AP and television networks by Edison Research. This includes preliminary results from a survey of 1,493 voters who voted early or absentee and were interviewed by landline or cellular telephone from Oct. 29 through Nov. 4. Results for the full sample were subject to sampling error of plus or minus 3 percentage points; it is higher for subgroups.