The 33-year-old will officially join Town at the end of his current contract on July 1st.
Speaking to the club’s official website he said manager David Flitcroft and the ambition of the club played a big role in him signing at the County Ground.
“First and foremost, the gaffer and his assistant have got a track record of success in this league and I’ve played against their teams in the league above,” Purkiss told the club’s official website.
“I know what they are about and they are driven, they are winners, they know how to win and they have a good track record of winning.
“That’s why I’m here. I’m here to have a successful season with one target in mind. The target this season will be to get promoted.
“There will be a lot of teams in the league who will want to get promoted but there’s a difference between saying you want to get promoted and putting that into action.
“The club are showing that ambition at the moment, there has been an investment in the playing squad, there has been an investment in the manager and there’s the commitment from the gaffer and his assistant who are moving down to establish a side in their image.”