The Girls Aloud star took on the role of Princess Fiona last year, replacing pregnant Amanda Holden.
Former Hollyoaks star Stenson is currently starring as Elle Woods in Legally Blonde, which ends its West End run on Saturday, April 7. She will take over from Walsh at the Theatre Royal Drury Lane from May 23.
Walsh, who made her West End debut in the role, will perform her final show on May 21.
Stenson will join stars Dean Chisnall and Neil McDermott - who recently made their debuts as Shrek and Lord Farquaad - and Richard Blackwood, who has played Donkey since the production opened.
Commenting on Twitter, she wrote: "Phew now I can talk about it! I will b playin Princess Fiona in Shrek the musical London frm may 23rd! Very excited always wanted 2b one ha! (sic)"
Shrek The Musical has extended its booking period at the theatre until March 31, 2013.
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