The open call for submissions asks users to submit their Halloween-themed items through the Steam Workshop. Entrants have until October 1 to submit their items.
"Scariness doesn't just happen, people. It's serious business," Valve said. "That's why we're giving you until October 1st to fill the Steam Workshop with horrifying Halloween-themed cosmetic items."
Users can also rate the Halloween items through the Steam Workshop to help determine which ones make the cut.
Last month, Valve released another large update for Team Fortress 2 that included the new Mann vs Machine co-operative mode.
Watch the 'Mann vs Machine' trailer for Team Fortress 2 below: