The Australian is the 15th Bulls player to pen fresh terms ahead of next season's campaign in the Championship.
Purtell joined Bradford from Sydney-based outfit Penrith Panthers in 2012.
"I see the one-year deal as a launchpad ahead of what is going to be a massive year for me personally," the 29-year-old told the club's official website.
"I cannot speak on behalf of the rest of the team but I am desperate to ensure an instant return to Super League next year. I feel as though we have a point to prove and things to put right."
Bradford play their final match of the season on Saturday when they travel to London Broncos.