Laura Tyrer is to replace Denise Van Outen in Chicago.
Tyrer secured the part of Velma Kelly in the musical via an audition that will be shown as part of new ITV series, The Big Audition, which begins next month.
Her previous West End credits include Charlie and the Chocolate Factory, and Matilda.
She joins the musical on September 24, and remains with it until October 6.
Van Outen had to pull out of a planned run in the show at short notice following an injury she sustained prior to rehearsals.
The Kander and Ebb musical is running at the Phoenix Theatre until January 5, 2019.