Our law enforcement sources tell us ... the NYPD's 1st Precinct was notified that an employee of The Dominick Hotel found a loaded firearm inside one of their rooms.

We're told officers were able to ID Polo as being the occupant of the room ... and our sources say it was registered under his name, Taurus Bartlett, and the DOB also matched up.

Manhattan District Attorney's Office executed a search warrant for the hotel room and found a loaded Glock 23 firearm inside the bedroom side of the hotel room.

We're told a firearm trace came back negative and the weapon wasn't flagged as stolen ... and an arrest warrant for Polo was drawn up as a result. NYPD officers arrested Polo around 12:50 PM Wednesday and charged him with Criminal Possession of a Weapon in both the 2nd and 3rd degree.

Cops also say Polo was carrying more than $23k in cash at the time of his arrest. It's been a rough few months for Polo and his family.

We broke the initial story in November ... Polo's caught a separate gun charge and his brother Trench Baby was arrested for murder in California.

To make matters worse, Polo's mother and sister just engaged in a very nasty spat on social media that started with a fistfight on Easter Sunday.

We just caught Polo in NYC this week ... he was genuinely excited about finally launching his "Hood Poet" album and promised big surprises along the way.