The supporter ran onto the pitch from the away end before striking the former Liverpool goalkeeper on his blind-side, leaving him on the floor and needing treatment.
The first Yorkshire derby between the two sides in five years was marred by the assault, which came just after Leeds had equalised through an excellent Michael Tonge effort on 80 minutes.
The fan escaped back into the mass of Leeds support before stewards or police could detain him, though pictures of his face have already been posted on social networking website Twitter.
"I'm embarrassed to be the Leeds manager tonight," Leeds manager Neil Warnock said after the game.
"He's a moron, an idiot. I don't mind rivalry but this guy should be made an example of, prosecuted and be put in prison.
"The guy who done this has the mentality of a five year-old. He's spoilt the night for everybody, as it was a good atmosphere here up to that point."
The attack from the fan followed relentless controversial songs and chants from the away end, and Wednesday manager Dave Jones felt they had been pushing the boundaries all night.
"Vile animals they are (the Leeds fans)," Jones said.
"The authorities have to do something, or these people will keep doing this. They were all chanting, so they shouldn't have been applauded afterwards.
"They want to come onto the pitch and punch my goalkeeper? They should be banned from every ground in the league."
Jay Bothroyd had opened the scoring for The Owls just before half-time.