Josh Robinson deflected header in the last minute to give Premiership leaders Crusaders victory over the Ports.
It was a drab encounter at Shamrock Park, with Ports keeper Billy Brennan pushing a cross onto the bar as the visitors dominated the first half.
There was late drama with Ports forward Mario Soares failing to connect at the back-post seconds before Robinson's close-range winner from a corner.
Crues keeper Sean O'Neill tipped over Gary Twigg's header in stoppage time.
Jordan Owens squandered the best opening of the first half with the Crusaders striker side-footing wide from Paul Heatley's cut-back.
It was a more even second 45 minutes but the chances dried up until a dramatic finale.
Substitute Soares was inches away from putting Portadown ahead, with only a touch required to divert a free-kick into net.
Crusaders were quick to capitalise as Robinson's header came off the unfortunate Twigg before finding the net.
Twigg almost equalised in the dying seconds but his powerful header was touched over by the alert O'Neill.
The win keeps the champions nine points clear of closest challengers Cliftonville.
What they said
Crusaders skipper Colin Coates said: "It was one of those days where you had to win your battles and I think we won seven out of 11.
"It wasn't a day for pretty football - it was about getting over the line and we did that.
"Cliftonville and Linfield are putting us under pressure but we'll just look after ourselves."