The TV and stage star will join Jason Manford, Beverley Knight and Adam Lambert on the panel for for the new series, hosted by Olly Murs.
The show will see members of the public transform into their favourite musical performers.
Discussing why she wanted to take part in the show, Smith, who has a one-year-old son, Billy, said:
“I guess part of it is having a son, you know I’ve always kind of shied away from talent shows before.
“I think maybe I was scared to step out of my comfort zone which was being an actress.
“It’s about making him proud and if I push myself out of my comfort zone, that’s good.
“I was very flattered to be asked to be honest because they could have had anyone but I’m loving it.”
Smith said she now hopes viewers see a different side to her, adding:
“It’s scary because I’m just not used to it. I get really nervous and I think most actors do when it comes to being themselves.
“I hosted another show Pooch Perfect but that was not filmed like this, it was very casual and you’re there all day surrounded by dogs which is my biggest love after my son.
“It’s very different to being sat on a panel with all that glam and the production. Hopefully I will be the eyes of the public and just say it how I see it.”
Smith described herself as “the easily pleased judge,” adding:
“I take my hat off to all of them [the contestants], I’m nervous just sitting in the chair and the stage is epic.
“These are people who are a mechanic by day and Elton John by night, to me it’s mind blowing that they can do that, so I’m in awe of them.
“Their families must be so proud, I’m just on my feet, I’m almost like a proud mum.
“I am so behind them all and I want them to do well, I know what a big thing it is to step out on the stage, in front of the nation as well.
“I guess as the actress, who has played roles like Cilla, I’m looking for more mannerisms and embodying the character, not just sounding alike.
“A lot of people can do caricatures of people whereas I’m kind of more about the little details and that’s kind of my role.”
Starstruck starts on Saturday at 8:30pm on ITV and ITV Hub.