Goals by Chris Ramsey and Gary Twigg gave Portadown their third league win in a row and heaped the pressure on Coleraine manager Oran Kearney.
Defender Ramsey headed in from Chris Casement's corner in the eighth minute.
Twigg ran on to home keeper David Miskelly's wind assisted free-kick to chip over keeper Wayne Drummond and double the Ports' lead on 22 minutes.
It was 10th-placed Coleraine's sixth successive defeat in the league while Portadown move up to fourth.
Ronnie McFall's Ports had gone into the game in confident mod after recent wins over Glentorsn and Ballymena United.
In contrast, Kearney's Bannsiders have been lacking confidence and they were in trouble early on at Shamrock Park as Ramsey headed the opener.
The corner came after Sean Mackle's goal-bound shot had been tipped over the bar by keeper Drummond.
Casement's outswinging kick was met by central defender Ramsey for his first goal of the season.
Things got worse for Coleraine when their defence misjudged Miskelly's long kick forward and Twigg's persistence was rewarded with a second goal for Portadown.
Coleraine had a chance just before half-time but Miskelly save Craig Moore's low shot.
In the second half Coleraine defender David Ogilby missed a chance to head in while Twigg glanced a header over for the Ports.
Then near the end, Coleraine striker Eoin Bradley crashed a shot off the crossbar