The 21-year-old defender, who played 41 matches for the Pirates last campaign, will remain at the club until 2016.
Manager John Ward is delighted to keep Parkes at the Memorial Stadium for a further year.
"I am really pleased that Tom's contract has been extended, as it shows the commitment of both the club and the player," he told the club's official website.
"He is a young man who has been made captain, and is setting an example for others. He has taken everything on board, and it has even improved his game.
"I am trying to strengthen and improve the side with the young players we have here, and Tom is a big part of that."
- Sports & Recreation