It is not perfectly legal to go topless in all state except the three stated. In WA it is not illegal but it is not legal either. Legal means you have the right to go topless. There is no law in WA state that says it is LEGAL.
Washington State Code
The statute used most often in the State of Washington for prosecuting cases of nudity (simple or otherwise) is the Indecent Exposure statute,
RCW 9A.88.010 and I’m going to quote part of it here as I think an understanding of this statute is very important as it affects nudists:
RCW 9A.88.010
Indecent exposure.
(1) A person is guilty of indecent exposure if he or she intentionally makes any open and obscene exposure of his or her person or the person of another knowing that such conduct is likely to cause reasonable affront or alarm. The act of breastfeeding or expressing breast milk is not indecent exposure.
In order to make a charge of indecent exposure, and to have it stand in court, all elements above have to be met:
- An OPEN and OBSCENE EXPOSURE
- That it was done INTENTIONALLY, and
- That it likely would cause reasonable AFFRONT or ALARM
Whether you intentionally get naked with the knowledge that it is probably going to cause affront or alarm, the expression still has to be an ‘obscene exposure’. There is a lot of confusion even in the law enforcement community over the difference between what we call ‘simple nudity’ and ‘obscene exposure’. For most police officers, just the fact that you are nude is often enough for them to consider it obscene and arrest you . . . leaving it to the courts and prosecutor to sort the charging elements out. Of course, it costs you time and money, and sometimes your reputation, to defend yourself.
Much more onerous is that a police officer in the State of Washington does not need a warrant to make an arrest for indecent exposure . . . just probable cause and a complaint. Most misdemeanor offenses are cited and the individual sent on his or her way with a promise to show up in court (
RCW 10.31.100: Arrest Without Warrant). Knowing that previous arrests for simple nudism under the Indecent Exposure statute rarely stand up (unless you plea the charge), the police have learned that as soon as they have one complaint this statute allows them to get the ‘offending’ person off the street with no repercussions. Immediate problem solved and the ‘perp’ is quietly released at the precinct a few hours later.
So before you go to the store in your birthday suit you better check your State, county and city laws!

It states the act of breastfeeding or expressing milk is not obscene, but I wouldn't doubt there are some things you could do while breastfeeding that could get you into trouble with the law as it is written.
https://nudehiker.wordpress.com/2008/01/29/nudity-and-the-law-in-washington-state/