A Florida man's OnlyFans shoot went from XXX to 911 ... after cops say he was caught pleasuring himself with his pants down in a public park.

Nathan Christopher Aleman was arrested around 5 AM Saturday in St. Petersburg after a bystander allegedly spotted him under a park pavilion touching himself, according to an arrest affidavit obtained by CBS12.

Deputies say the witness confronted Aleman ... and instead of making a run for it, he apologized and agreed to hang around until cops arrived.

And apparently there was a reason for the very public performance.

Investigators say Aleman admitted he had exposed himself ... telling deputies he was in the middle of filming content for OnlyFans when the unsuspecting bystander sat down nearby.

So, yeah ... dude allegedly turned the park pavilion into his own personal porn set -- without exactly booking the location first.

Aleman was booked into the Pinellas County Jail on a charge of first-offense exposure of sexual organs.