The former Hoops hitman has completed a three and a half year deal with the Danske Bank Premiership side and is happy to have left Tallaght Stadium 'on good terms'
Twigg had spent four seasons at Tallaght Stadium where he scored 89 goals and won two Airtricity League Premier Division titles, but has made the decision to leave the club due to family commitments.
The former Brechin City striker revealed that he had agreed to join the Ports during the summer, but remarked that it was 'right' to depart the Hoops 'on good terms'.
He said: “I agreed to join the club in the summer and wanted the news to come out earlier, but for obvious reasons it was right to leave Shamrock Rovers on good terms.
“The manager and chairman are very positive and assured me of their desire for success for the club and I want to be a part of that and win trophies.”
Portadown boss Ronnie McFall admitted that the Shamrock Park club had been targeting the striker for a long time and believes that the signing of Twigg, who finished this season as Airtricity Premier Division top goalscorer, is 'a boost for the whole league'.
“We have been chasing Gary for a long time," he said. "It goes without saying that when someone of his quality comes along, we will try to sign him. His signing is a boost for the whole league.”
Portadown find themselves in eighth position in the Danske Bank Premiership, with 16 points from 13 league games.