King County will be offering 12 weeks of paid parental leave.
The County Council unanimously approved of King County Executive Dow Constantine's proposal during a meeting on Monday.
After the council vote, Constantine said the move demonstrates the county's commitment to recruiting and retaining its workforce.
King County is not only the state's most populous county, but it's also among the top 10 largest employers in Washington.
Supporters of the measure say the change will give employees more time to connect with their newborn children.
Other large employers, including Amazon and the Gates Foundation, have similar policies. So does the City of Seattle, although the city's plan covers four weeks. That's a much smaller time span.
Seattle City Councilmember Kshama Sawant had called for extending it to 12 weeks, but her plan did not have enough support to pass this year.
King County's proposal takes effect next year.