Shaquille O'Neal was back on the set of "Gravesend" Tuesday ... filming scenes for the mob drama while looking far more like a crime boss than a lovable basketball legend.

Check out these images TMZ Sports has obtained ... the 15-time NBA All-Star was decked out in a huge gold chain, a black track suit and some specs as cameras rolled on him for his part in the Amazon Prime series.

Playing "Mustafa," he chatted away with businessman and producer, Jas Mathur, who also has the role of "Miguel" in the hit show.

The two appeared to be in deep talks in one of the scenes -- and while it's unclear exactly what was said between the actors ... given the way the show features shady business deals and organized crime, it's safe to assume NBA free agency was not one of the topics.

The show kicked off back in 2020 ... but Shaq's debut will come in the program's upcoming Season 3.

He told News12 last year he actually got involved in it all after he informed "Gravesend" director and star William DeMeo the show was one of his favorites.

"When I contacted Will just to tell him I was a fan, he asked me if I wanted to be in season three," he said. "So, season three is going to be very, very exciting."