Another day, another fan who doesn’t know boundaries ... this time, one shameless concertgoer took things way too far by reaching up to grab Halsey ’s butt -- mid-performance!

As you can see, Halsey was performing "Is There Somewhere" in Washington, D.C., earlier this week when the unidentified fan in the crowd reached dangerously close to her thigh ... then brazenly slid a hand under her leather skirt, copping a quick feel in front of everyone.

The disturbing up-close clip -- shot from right behind the creep -- shows a gloved hand, presumably security, finally stepping in to swat away the paw ... but TBH, it takes way too long to happen.

It’s a super uncomfortable watch, but looks like Halsey didn’t even notice what went down and just kept the show rolling.

The clip’s blowing

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