Ja Morant Allegedly Flashed Gun, Punched Teen After Pickup Game Fight

New troubling allegations surrounding the alleged pickup basketball brawl between Ja Morant and a teenage boy in July ... with the young hooper claiming the Memphis Grizzlies superstar flashed a gun at him during the altercation.

TMZ Sports broke the story in January -- Morant is accused of punching a 17-year-old boy after getting in an argument during a game at Ja's residence in Tennessee ... which resulted in a lawsuit filed in September.

According to the docs, the boy claimed he "accidentally" hit Ja in the face after throwing the ball in his direction ... and the All-Star guard retaliated by hitting him with a closed fist and knocking him to the ground.

The Washington Post reported Wednesday that not only did the teen accuse Morant of punching him at least 12 times, resulting in a "large knot" on his head ... but the 23-year-old also went into his house and returned with a gun in his waistband -- with his hand on the weapon.

The boy told detectives Ja's dad, Tee, then yelled at him ... telling him to go back inside.

When Morant was questioned about the incident, police mentioned the boy's claims of exposing a weapon, but did not explicitly ask if he had a gun. The outlet notes Morant and his attorneys did not deny the claim.

As for the altercation, Morant told detectives he did, in fact, hit the teen ... but claimed it was out of self-defense after he was hit with the ball. In his eyes, "The ball was the first swing to me."

As we previously reported, Morant filed a police report of his own a few weeks later ... claiming the teen threatened to "come back and light this place up like fireworks" and he feared for his safety.

Police investigated the allegations for several weeks ... before they submitted the case to the district attorney's office on Oct. 4. Less than a month later, cops say the D.A. declined prosecution, citing a lack of evidence.