A Bronx man has been arrested for murder in the death of a Bronx baby who had a fatal mix of fentanyl and heroin in his system, police said Saturday.

Chayanne Mendez Rodriguez, 29, is the second suspect to be charged with murder for the Dec. 27 death of one-year-old Darwin Gonzalez-Santana, officials said.

Mendez Rodriguez was one of three people in the home — including the child’s parents — when the boy died, according to police.

Baby Darwin was rushed to St. Barnabas Hospital after police responded to a report of an unconscious baby on University Avenue in Morris Heights.

He had no obvious signs of trauma, but investigators found drugs inside the family’s home, cops said at the time.

Police ruled the tot’s death a homicide in March, and a month later the medical examiner found that the baby had traces of fentanyl and heroin in his system at the time of his death.

In May, Darwin’s mother, Daira Santana-Gonzalez, 23, was charged with her son’s murder.

Investigators are still looking for the baby’s father, who they believe fled to the Dominican Republic, law enforcement sources said.

Police nabbed Mendez Rodriguez at the airport Saturday morning, where he was attempting to catch a flight to the Dominican Republic, authorities said.

He was being held at the 46th Precinct Saturday awaiting arraignment.