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Ariana Grande said thank u, but no thank u to Forever 21 when they pitched her an endorsement deal, but the clothing store went and launched a ripoff Ariana ad campaign anyway ... according to a new suit.

The singer alleges Forever 21 and her team were in talks for a deal in December 2018 into January, but the company wouldn't pony up enough dough to use her name and likeness.

Instead, Forever 21 allegedly stole it from Grande for a campaign across their website and social media ... and also hired a look-alike model to make it seem even more like the real deal.

According to the docs, obtained by TMZ, F21 published at least 30 unauthorized images and videos misappropriating Ariana's name and likeness, and did so by piggybacking off of the huge success of her "Thank U, Next" album.

Grande claims the company stole imagery from her album to create a false perception Ariana was backing the Forever 21 brand. The suit alleges F21's look-alike model also reenacted scenes and shots from the album's singles and music videos ... specifically "7 Rings."

The store Riley Rose, which was launched by the daughters of Forever 21's founders, was also allegedly involved in the ripoff campaign.