A sexual assault lawsuit against Steven Tyler has been dismissed because a judge says the alleged incident did not present a serious risk of harm to the plaintiff ... TMZ has learned.

Here's the deal ... last year the singer-songwriter was sued by Jeanne Bellino who alleged Steven kissed, groped and began "humping her pretending to have sex" twice one night in 1975 when she was 17.

Bellino brought the lawsuit against Tyler just last year via New York City's Victim of Gender-Motivated Violence Protection Act -- a law extending the statute of limitations for certain alleged sexual misdeeds, in some cases from decades past, to finally be heard in court.

But, to take advantage of the longer statute of limitations, the judge said the alleged act needed to meet certain criteria ... including "presenting a serious risk of physical injury" -- a box the judge in Bellino's case says this alleged incident does not check.

Basically, what the judge said in the decision handed down Wednesday is that Tyler allegedly forcibly kissing and dry-humping Jeanne -- as she claims -- might be distressing, but didn't rise to the level of presenting any risk of serious injury, so he tossed the suit.

It's a pretty noteworthy decision, and unclear what future ramifications the precedent set by the judge today may have on other cases filed in New York City under the same provision.