'Euphoria' Star Sydney Sweeney Sued for Allegedly Bailing on Swimsuit Deal

Sydney Sweeney broke an agreement with a swimwear brand to promote their products, then turned around and wore the suits on "Euphoria" ... this according to a new lawsuit.

The HBO star is being sued by the swim and lifestyle brand LA Collective for allegedly breaking her contract to model and promote the company's swimsuits.

According to docs obtained by TMZ, the fashion brand claims it reached an agreement with Sydney last year, where she would be the influencer for the launch of their new line, "Somewhere Swimwear."

The brand says Sydney initially appeared to be on board and approved all their designs and production prototypes ... but they claim her camp suddenly canceled the agreement without any explanation or cause, refusing to proceed.

And, get this ... the brand claims Sydney then appropriated the designs for her own use, wearing the swimsuits on at least 5 episodes of "Euphoria."

The swimwear company says they expected to sell more than $3 million worth of suits under the deal and want the actress to cough up big damages for backing out.