Daniel Penny -- the man who choked Jordan Neely to death on an NYC subway -- has been indicted by a grand jury.

The ex-marine has now been formally charged as a result of his arrest last month -- when he was booked on a 2nd-degree manslaughter charge ... it's not yet clear if the indictment charges are the same ones he was booked on.

Like we said ... the exact charges in the indictment are unknown at this point. It appears they’ll remain under seal until he appears in court at a later date.

There's been a lot of discourse over Penny's arrest. Some see it as justice -- including Neely's family, who want this guy prosecuted to the full extent of the law ... saying he acted as a vigilante that day and took things way too far with his prolonged chokehold.

Others, however, are saying Penny shouldn’t face charges for acting as what they describe as essentially a good Samaritan. There are reports Neely was harassing folks down in the subway that day ... and that Penny stepped in to protect others.

Of course, there's a race element that's been floated as well. Neely was Black, and Penny is white -- and there are folks who believe his actions may have been motivated by the color of Neely's skin ... although, Penny has strongly denied that, insisting he was just trying to do the right thing.

Regardless, he’ll now he'll have to navigate the criminal justice system. It's unclear when he's due in court again. It goes without saying ... Neely's death has sparked protests, and in some cases -- things have gotten tumultuous with cops.