A Bronx man was busted tonight for beating his girlfriend’s 1-year-old son to death, police said.

Reginald Smith, 24, of Wakefield, allegedly killed little Richard Flood Wednesday morning at the mother’s apartment on East 232nd Street.

An autopsy showed the tot died of blunt-force trauma that caused deadly damage to his internal organs, the Medical Examiner’s Office said.

The boy also had bruises, burns and a cast on his leg from previous injuries, ­authorities said.

Police found the baby after a 911 call and rushed him to Montefiore Hospital, where he was pronounced dead.

The mother was not home at the time of the attack.

Cops arrested Smith Wednesday on an outstanding warrant for disorderly conduct and questioned him in the baby’s death.

Smith maintained that the baby fell while he slept, but authorities said they have enough evidence to charge him with murder.