An off-duty NYPD cop was pummeled by an angry commuter at Grand Central Terminal early Wednesday morning after the pair accidentally bumped into each other on a stairwell, cops said.

The officer was traveling to work in plain clothes when he was repeatedly slugged in the face by the vengeful straphanger in the Grand Central–42nd Street station at around 6 a.m., according to police.

The unnamed cop had just exited a No. 7 train and was walking up the packed staircase to catch a Bronx-bound No. 6 train when he and another man – who was going down the stairs — bumped into each other. 

The other straphanger allegedly snarled, “Why the f–k did you bump on me?” 

“It’s crowded and I’m trying to catch the train,” the off-duty cop replied. 

The agitated commuter then unleashed multiple blows on the victim’s forehead, nose and face — causing substantial pain, redness, bruises and swelling. 

The suspect continued walking down the stairs and fled, cops said. 

The victim – who was unarmed and did not identify himself as a cop – also continued on his way and boarded a northbound No. 6 train, when he noticed blood on his clothing, police said. 

When the officer arrived at the 43rd Precinct for work, he received medical attention and was taken to Jacobi Medical Center, where he was listed in stable condition, police said. 

No arrests were made on Wednesday afternoon.