A Brooklyn teen accused of pummeling a 30-year-old man aboard a southbound R train Sunday afternoon surrendered to cops Monday morning.

Lamaj Rustem, 18, of Bay Ridge was arraigned in Brooklyn Criminal Court Monday night, before Judge Sharon Hudson where he was charged with assault, menacing, and harassment in the Oct. 22 attack on victim, Anthony Hamoui.

The defendant, voluntarily surrendered to cops at the 72nd precinct Monday morning with his attorney, Lance Lazzaro.

Brooklyn Assistant District Attorney Tziyonah Langsam said the victim – who remains in critical condition at Lutheran Medical Center – went into cardiac arrest following the brutal beatdown and has a pulmonary embolism, is unable to breathe on his own and “may die of his injuries.”

Rustem was released on 15,000 bail, and left court with his hoodie pulled over his head surrounded by family members.

Rustem is due back in court on October 27th.