(NSFW) Is it legal? (Details inside)

No.

If I had to guess I would say no, but maybe legal consul is needed.

A public restroom is a public place. Illegal.

No. If it illegal for a senator to tap another man’s foot in an adjacent stall, then if I were you I definitely wouldn’t be pulling taffy in there.

Yep, it is illegal to perform sexually explicit acts in public, period.
The question is WHY would someone feel the need to jerk off in a public restroom to begin with. Jeez, if you can’t drive your car or leave home without the urge to purge your monkey well… take some salt peter. lol

Some people do have conditions that require frequent attention.
If he is playing 5 on 1 in a bathroom stall, one can only hope he is leaving a clean court.

What in the world prompted this question?

@Dutchess_III The South Park episode where the TSA watches people in the bathrooms to make sure that everyone is safe.

In the US, yes. In Canada… probably, but different jurisdictions have different laws, and Canada is weird, so I can’t say for certain without research.

If the door is closed and the person is quiet why would anyone be bothered by it? It’s not the kind of place I would choose to masturbate but if no one can see then I don’t see the problem. Of course my answer has nothing to do with the “legality” of such an act, I have absolutely no idea how these things are viewed by the law in the US.

As long as no one sees, hears or catches you in the act then you should be OK. But word to the wise, you’re better off keeping your extracurricular activities in your own home. No harm, no foul. No one wants to hear you, or anyone else, jerking off in a bathroom stall.

Should it be?

The US and Canada both extend an expectation of privacy to public restrooms. Police officers in the US, for example, cannot peek into a bathroom stall to see if you’re doing anything illegal (due to both the Fourth and Fourteenth Amendments).* Strictly speaking, then, it is perfectly legal to masturbate in a public restroom in either country.

What makes it complicated, however, is the circumstances of getting caught. As already noted, you can’t be prosecuted if someone peeks in on you. In fact, you could technically prosecute them for invading your privacy. But it’s unlikely that would ever work out in your favor because all the peeper has to do is claim that you made a noise that caught their interest.

That would get you in trouble because your expectation of privacy only applies so long as you’re keeping to yourself. As soon as you make what you’re doing known to anyone else, it counts as public lewdness. This doesn’t matter with ordinary bathroom noises because they aren’t considered lewd under the law. But if you do anything to make a “lewd” act public, then what you are doing is no longer considered legal.**

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* There’s a surprising amount of US case law on this issue, by which I mean both restroom privacy in general and restroom masturbation.
** Note that the above answer only applies to the situation described in the OP. If the restroom is not designed in such a way as to give privacy in the first place—e.g., if the stalls lack doors or the doors do not close properly—then no expectation of privacy exists.

Well….it’s illegal to crap in public, too, which is why we have the stalls.