Under the new law, people there can now choose to have their body turned into soil after their death.
The process is seen as an alternative to cremations and burials, and as a practical option in cities where land for graveyards is scarce.
At the end of the composting, loved ones are given the soil, which they can use in planting flowers, vegetables or trees.
The bill was signed into law by Governor Jay Inslee on Tuesday.
Katrina Spade, who lobbied for the law to be introduced, founded a company that could be the first to provide the service.
"Recomposition offers an alternative to embalming and burial or cremation that is natural, safe, sustainable, and will result in significant savings in carbon emissions and land usage," Ms Spade told Agence France-Presse.