A 65-year-old man is dead after a witness says he got into an altercation with an 18-year-old at the Jay Street-Metrotech subway station Wednesday afternoon.

The incident happened around 2:30 p.m. on the platform of the R train.

Police say a witness told them the teen was talking to himself and got in the personal space of the 65-year-old. The man then asked him to move away, which he did for a short time.

The teen then came back and sucker-punched the man, News 12 is told. The man then fell onto the tracks and was later pronounced dead at Brooklyn Hospital.

The man was not hit by a train or electrocuted. His official cause of death is unknown at this time.

The 18-year-old was taken into police custody, and News 12 is told he has nine prior arrests for crimes including criminal mischief, robbery and possession of a weapon.

Police are asking anyone with more information about the incident to come forward.