A 21-year-old man has pleaded guilty to vomiting on another spectator and his 11-year-old daughter in the stands during a baseball game.

Matthew Clemmens pleaded guilty to one count each of simple assault, disorderly conduct and harassment for his conduct during a game at Citizens Bank Park in Philadelphia on April 14.

Clemmens stuck his fingers down his throat and vomited on Michael Vangelo, an off-duty Easton police captain, and one of Vangelo's daughters after Clemmens's companion was ejected from the park, Assistant District Attorney Patrick Doyle said.

Clemmens and his friend were spilling beer, swearing and heckling Vangelo and his daughters from the time they arrived at their seats, according to a statement of facts read in court.

Vangelo's 15-year-old daughter asked the pair to stop the profanity, and Vangelo complained to security that Clemmens's friend was spitting, with some of it hitting his 11-year-old daughter, Doyle said.

After the friend was ejected, Clemmens was sitting alone behind the Vangelos when he answered his mobile phone and said, "I need to do what I need to do. I'm going to get sick," the prosecutor said.

Clemmens then put his fingers down his throat and threw up on the father, with some vomit splashing on to Vangelo's younger daughter, Doyle said.

He then punched the father several times in the head before other fans in the stands subdued him, the prosecutor said. He screamed expletives at the crowd as he was led out of the park.

Clemmens's mugshot showed him with a swollen black eye, and authorities acknowledged he was hit as he was being subdued. No-one else was charged in the case.

In exchange for Clemmens's guilty pleas, charges including reckless endangerment and corruption of minors were dropped.