Robert Fitzgerald, 24, admitted invading the pitch after Stevenage's 3-1 win against Newcastle in the FA Cup on January 8.
He also admitted punching Stevenage defender Scott Laird as crowds of fans celebrated the win on the pitch.
The court heard Fitzgerald, a Stevenage fan, threw the punch "out of loyalty" to his girlfriend Jade Coles, who had previously been in a relationship with Mr Laird.
Jennifer Browne, defending, said: "Mr Fitzgerald goes onto the pitch, he congratulates other players. He sees the injured party and he throws a punch. Why? Because the injured party had had a relationship with Mr Fitzgerald's current girlfriend.
"This was some time ago but my instructions are that the injured party did not treat the current girlfriend, Mr Fitzgerald's girlfriend, correctly.
"Mr Fitzgerald takes matters into his own hands and it was seizing the opportunity. It wasn't premeditated to go on to the football pitch."
He will be sentenced on February 17.