Washington is the first state in the United States to legalise the recreational use of marijuana.
A new law makes it legal for people over 21 years of age to possess and use up to 28 grams of the drug.
A number of states have already legalised pot for medicinal purposes, but Washington is the first allowing it purely for fun. Within a year it will be legally sold and taxed at licensed shops.
However, federal law classifies marijuana as an illegal narcotic.
Midnight pot parties took place across the state, with some 200 people gathering outside the Space Needle in Seattle.
The ABC reports officers took no action and the Seattle police department said that for the moment it will issue only verbal warnings.
Colorado voters approved a similar law on 6 November, which will take effect there on 5 January.