3:33 PM PT -- Sources close to the situation tell TMZ ... Liam Payne and his girlfriend Kate Cassidy arrived in Argentina on September 30, posting several social media posts together showing them having fun on vacation. We're told she left Argentina on the 14th. Liam stayed behind, and checked into the CasaSur Palermo Hotel.

We're told someone at the hotel called 911 to report an "aggressive" male at the hotel today ... which lines up with what witnesses told us about Liam's behavior in the lobby.

Liam Payne is dead after a fall from his hotel balcony in Argentina ... TMZ has learned.

The former One Direction singer died Wednesday evening. Several witnesses at the hotel where he was staying in Buenos Aires tell us Liam fell from his room on the 3rd floor of the CasaSur Palermo Hotel. We do not know if it was intentional or accidental.

TMZ has seen a photo showing Liam's body on the deck at the hotel with tables and chairs nearby. You can clearly see his tattoos -- a clock on his left forearm, and a scorpion on his abdomen -- which helped us confirm early reports from witnesses.

We're told this happened shortly after 5 PM in Buenos Aires, and that Liam had been acting erratic in the hotel lobby earlier -- he smashed his laptop, and had to be carried back to his room.

Police and other first responders are all around the hotel now ... and authorities have put up a red tent over the body as they begin their investigation into Liam's death.

He had been in Argentina recently for a Niall Horan concert earlier in the month, and the 1D bandmates actually reunited at the show.

Liam was only 16 years old when he joined One Direction ... and the group became the biggest boyband in the world after their 2010 launch on "The X Factor." The band ultimately broke up in 2015, and he was considered one of their main songwriters.

As we reported ... Liam revealed in June 2021 that he struggled with alcohol and prescription drug addictions at one point when One Direction was on tour, and things got so bad he had "severe" suicidal thoughts.

Earlier this week, there were reports about Liam's ex-fiancée Maya Henry alleging he'd left her after asking her to get an abortion. Her attorneys had been trying to send him a cease-and-desist letter to stop contacting her.

It's unclear, at this point, if that triggered his erratic behavior at the hotel.

Liam was only 31.

RIP