The ball is in Jill Zarin's pickleball court ... because a lawsuit has just been filed against her and her boyfriend, Gary Brody.

Noah Springer is suing Zarin, Brody and GNG Enterprises ... with Springer claiming he invested $500K back in 2022 in exchange for a 25% share of a company called Pickle Pro Labs, LLC. He says he was promised a monthly salary of $5K while he and the company developed a machine called the Go-No-Go.

Around May 2024, Springer says he stopped getting paid ... and, around the same time, he claims Zarin and Brody founded GNG Enterprises -- a new company -- without his knowledge.

Through this company, Springer says Zarin and Brody have been selling the Go-No-Go ... a product he says was "conceived, engineered, researched, funded, and developed by Pickle Pro Labs" using company resources.

Springer says the pair were not authorized to sell this machine under the GNG banner ... but he says they did so anyway without permission. He's suing for damages and wants an injunction against Zarin and Brody that blocks them from diverting Pickle Pro's money and business opportunities to GNG.

We've reached out to Jill and Garry ... so far, no word back.