A homeless man was arrested for allegedly randomly attacking a straphanger with a baton at Grand Central last week, police said.

Frankelly Grullon, 37, allegedly approached a 63-year-old man in the mezzanine area of the Grand Central–42nd Street station around 1 a.m. Wednesday and began to argue with him, cops said.

When the victim tried to walk out of the station, Grullon allegedly hit him about the head with an expandable baton, according to the NYPD.

The subway-rider was brought to NYU Langone Health hospital in stable condition, cops said.

Grullon, who police said is “undomiciled,” was arrested Friday and charged with assault.