The TV stars teamed up for a stirring rendition of I Know Him So Well from the musical Chess, which is on Holden’s new album Songs From My Heart.
Sharing a clip on Instagram of the pair performing, Britain’s Got Talent judge Holden wrote: “I Know Him So Well - Out Now! On my album SONGS FROM MY HEART.
“This was one of the first ever singles I ever bought (I gave it to my mum, who loved the song!).
“Thank you @sheridansmithster, it was wonderful to share this moment with you, every performance you give is so powerful and honest.”
“I hope you all like it,” Holden added to her 1.5 million followers on the site.
The BGT judge, 49, also shared a photograph of her and West End star Smith smiling, with their arms around each other.
“I’ve adored @sheridansmithster for years and we’ve always hugged when we met but we didn’t know each other very well,” she said.
“I really wanted her to duet with me on ‘I Know Him So Well’ so I just asked her.
“She was heavily pregnant, but she couldn’t have been more enthusiastic or wonderful when I called her, and the final result is better than I even imagined.”
Holden is set to unveil Songs From My Heart in October.
Speaking on her Heart Radio show earlier this month, she said: “This album is all the songs that I've loved in my life, sung for the people that I love in my life who mean the most to me.
“It's a massively emotional album and hopefully it’s something that everyone can relate to."