Forster scored two tries in his side's quarter-final triumph at Ebbw Vale, but missed the trip to Bridgend last weekend because of a shoulder injury. Now he is named in the side alongside Matthew Veater and Steve Ojomoh in the back row.
Also fit to return is former New Zealand scrum half Ofisa Tonu'u. He takes over the No 9 jersey from David Llewellyn after recovering from a hamstring strain against Ebbw Vale.
Bridgend already have three wins to date over Newport this season, while in the Cup Bridgend lead 2-1. They beat the black and ambers 7-18 in 1974-75, and 16-18 in 1992-93, but lost 9-4 in 1991-92.
"Bridgend gave us a good hiding last weekend. We were off colour and they played very well and deserved their 56-30 victory," said Newport captain Simon Raiwalui.
"We've talked about that defeat as a group of players and we're determined it won't happen again. The Millennium Stadium is a great place to play and I'm sure we will raise our game.
"While we lost badly last weekend, we had two very close games against Bridgend at Rodney Parade earlier in the season when we should have won. Saturday is a one off game, with a place in a Cup final at stake, so I'm sure we'll be up for it."
NEWPORT RUGBY: Percy Montgomery; Alex Cadwallader, Jason Jones-Hughes, Jon Pritchard, Luke Nabaro; Shane Howarth, Ofisa Tonu'u; Rod Snow, Paul Young, Chris Anthony, Simon Raiwalui (captain), Ian Gough, Matthew Veater, Jason Forster, Steve Ojomoh.
Replacements (from): Adrian Garvey, James Richards, Mike Voyle/Andy Powell, Rhys Jones/Gareth Gravell, David Llewellyn, Jason Strange, Nathan Brew.