The 21-year-old, who will now remain with the Pirates until at least 2016, has been praised by his manager John Ward.
"I am really pleased that Tom's contract has been extended, as it shows the commitment of both the club and the player," Ward 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."
Parkes played 41 times for the club last season.