The disgraceful incident took place in the second half of the goalless draw at Fratton Park, which saw the Black Cats advance to the final 1-0 on aggregate
A Portsmouth fan appeared to kick and punch Sunderland’s Luke O’Nien during the League One play-off clash at Fratton Park on Thursday night.
In the second half of the semi-final second leg tie, O’Nien and Portsmouth defender Tom Naylor tussled on the sideline before the Black Cats full-back was forced over the edge of the advertising hoardings.
While O’Nien lay on the ground, a fan in the crowd wearing a grey jumper twice appeared to kick the 24-year-old before seemingly then swinging a punch.
Players from both sides rushed over along with referee Peter Bankes, with Max Power and Lee Cattermole confronting the home supporters.
That prompted an angry exchange, with Bankes having to intervene and order the Sunderland players away.
The referee then went over to speak to the fourth official, John Brookes, before making his way over to a have a word with a steward.
Police were then called to the scene, with officers stationed close to where the incident took place just a few minutes later.
The supporter in question was allowed to remain in his seat for the remainder of the match but the matter may yet be investigated by the club and police.
Sunderland went on to book their place at Wembley, with the game ending goalless to earn them a 1-0 victory on aggregate.
They will face the winner of Friday night’s other semi-final, between Charlton and Doncaster, in the League One play-off final.