Howson joined Curzon Ashton in the summer after several seasons with Salford City.
The defender previously played for Prestwich Heys, Radcliffe Borough, Australian side Bunbury Force and Ramsbottom United before joining the Ammies in 2015.
Southport manager Kevin Davies told his club`s official website: “We played Curzon Ashton recently and we were really impressed. He’s aggressive, he brings leadership and experience to the club.
“He’s a great personality to have in and around the squad. We’ve seen that already and we know that from talking to those that know him. We’re delighted to bring Steven to the club and I’m sure he’ll be a really positive addition”.