Grace O'Marcaigh just filed the suit against King in L.A., alleging she was working as a steward on a yacht she claims Diddy himself chartered a few days before New Year's Eve in 2022.

In the suit, O'Marcaigh alleges she was doing her job during the shindig -- which she claims was advertised as a family-friendly excursion -- and King came on, after which she claims he immediately became fixated on her, uncomfortably so.

She alleges King had her take a shot of liquor, and she claims she immediately suspected it was spiked ... but despite that, she was able to tell him to get off of her and leave her alone. She claims King was trying to kiss and grope her, despite her protestations.

O'Marcaigh's lawsuit purports to describe audio recordings of what she claims depict her telling King she's not interested in sex with him and trying to get away from him while they were together in a studio on the yacht. Eventually, she says she was able to get away -- but claims King insisted she find him a place to sleep, so she says she led him to the theater.

Once there, O'Marcaigh claims King cornered her again and attempted to force her to perform oral sex on him -- but she says she fought him off, and includes in the docs photos of her forearm, which she claims was bruised in the altercation.

Eventually, O'Marcaigh says she was able to escape ... but claims to have suffered mentally and emotionally ever since. BTW, she claims Rodney Jones -- Diddy's formerly-hired music producer, who's also suing Diddy -- was in the mix for this yacht party, and she's being repped by Tyrone Blackburn, the same lawyer who filed Rodney's lawsuit.

Diddy is named as a defendant in this lawsuit as well ... accused of being liable for actually chartering the yacht. O'Marcaigh also claims to have witnessed Diddy onboard with women she calls "sex workers" ... and that Diddy exposed himself to Yung Miami while they played a game of "Careesha Please." Interestingly, she says that was all recorded by a Hulu camera crew.

O'Marcaigh is suing Diddy, specifically, for aiding and abetting King in the alleged assault.

Blackburn's filing of O'Marcaigh's suit comes 2 days after he himself got into some legal hot water. He will have to go before the Grievance Committee of New York's federal court system for improperly filing cases in federal court "to garner media attention, embarrass defendants with salacious allegations, and pressure defendants to settle quickly." Worst case scenario is the Committee could recommend he be disbarred.

Diddy's attorney Aaron Dyer tells TMZ ... “We have not seen this woman’s claim but I’m sure we can expect the same kind of manufactured lies we’ve come to expect from Tyrone Blackburn and his clients, just as we saw in Rodney Jones’ lawsuit – which has yet to be served." He adds, they learned about the lawsuit "the same way anyone hears about Mr. Blackburn’s filings: through the media.”