Costelow replaces Ioan Lloyd at ten with in form scrum-half Gareth Davies his half back partner. Lloyd shifts to full back with Ioan Nicholas dropping to the bench.
There is a change in midfield with 19-year-old Macs Page coming in to partner 22-year-old Eddie James.
Blair Murray and Tom Rogers, who both scored stunning solo tries against the Bulls, continue on the wing.
Up front, Peel has made two changes to the pack.
At loose-head, Scotland international Alec Hepburn swaps places with Kemsley Mathias and he packs down alongside Wales squad members Ryan Elias and Henry Thomas.
Jac Price, who impressed off the bench last weekend, is handed a start alongside Sam Lousi in the second row, while the back row of Max Douglas, skipper Josh Macleod and Taine Plumtree is unchanged.
Jarrod Taylor comes into the match-day 23 to take his place among the replacements.
Scarlets’ nail-biting victory over the Bulls last Friday ended a five-match losing run at Parc y Scarlets.
They are now 10th in the table having won their last two URC fixtures and have not won three in a row since February 2023.
Italian side Zebre lost by the narrowest of margins at home to Lions last Saturday, their first single point defeat in the competition since they lost at Parc y Scarlets in November 2020.
They have not won away from home in URC since a visit to Treviso in January 2021 and have not beaten a Welsh region in the Championship since Dragons visited Parma in April 2022.
With four defeats from their five games Zebre are 15th in the table, above only Ospreys.
Head coach Dwayne Peel said:
“We were obviously delighted with the win over the Bulls, but the focus turns quickly to Zebre who are a very dangerous side if you give them opportunity.
“They have good quality runners and like to throw the ball around. It is another important game for us and hopefully we can build on the momentum and confidence we have generated with our last two wins.”
Scarlets XV to face Zebre:
Ioan Lloyd, Tom Rogers, Macs Page, Eddie James, Blair Murray, Sam Costelow, Gareth Davies
Alec Hepburn, Ryan Elias, Henry Thomas, Jac Price, Sam Lousi, Max Douglas, Josh Macleod (captain), Taine Plumtree.
Replacements:
Marnus van der Merwe, Kemsley Mathias, Sam Wainwright, Alex Craig, Jarrod Taylor,
Efan Jones, Ioan Nicholas, Johnny Williams.
[Lead image: Scarlets Rugby]
