New York City Mayor Eric Adams is now facing federal criminal charges, after a grand jury indictment ... TMZ has confirmed.

Adams, now the first NYC mayor to ever be charged while in office, is planning to surrender next week, according to the NY Post. The charges, which are currently sealed, are in connection with a federal probe, and the indictment is expected to be unsealed Thursday morning.

Adams told the NY Post, "I always knew that if I stood my ground for New Yorkers that I would be a target -- and a target I became. If I am charged, I am innocent, and I will fight this with every ounce of my strength and spirit."

The historic grand jury indictment was first reported by the New York Times -- but with the case still sealed, it's unclear exactly what the allegations are against Adams.

However, the NY Post is reporting the case is connected to allegations the Turkish government illegally funneled money into Adams' mayoral campaign.

It's interesting, Mayor Adams is being prosecuted by the same U.S. Attorney’s Office that's trying to nail Diddy ... the Southern District of New York.

The 2 men have some recent history -- Adams demanded Diddy return his Key to the City after the Cassie beating video came out, and back in June, he did just that.

Story developing ...