Porritt Tweeted on Wednesday afternoon: "It’s been a long/short period at @safc but it’s time to move on.
"Fantastic club with amazing loyal fans shame how it ended but I wish everyone at the club the best."
Porritt's time at Boro was ruined by an injury he received in summer 2020.
The former Guiseley and Hyde United ace was forced off in the dying stages of Boro’s 1-0 win over Eccleshill United - suffering a broken fibula, torn ankle cartilage and detached muscle from his ankle. He had only joined Boro towards the end of the previous season and had started to impress in pre-season before the injury.
Harry Coates who also left Boro last week, has signed for their NPL Premier Division rivals Ashton United.
Ashton boss Michael Clegg, speaking on the club's Twitter account, said: "Harry first impressed me when he was at Hyde, he is a commanding centre half. He is a good age, a leader and a real handful in both boxes.
"Two seasons ago I played against Scarborough three times in a couple of months due to switching over to Ashton. In a couple of the games, he didn't play, then the difference when he did I thought was huge. So ever since that he has been someone we have tracked from afar."