The actress and the singer, both 51, sang the Elton John hit, which appears in Kidman's movie Moulin Rogue!, before attending the Big Little Lies Season 2 premiere Wednesday.
Kidman shared a video of the moment on Instagram. The clip shows Urban playing the piano and singing as Kidman smiles and sings along.
"A little backstage moment before heading to Monterey @KeithUrban," Kidman captioned the post.
Urban supported Kidman at the premiere Wednesday in New York. Kidman spoke about the singer in an interview with Entertainment Tonight on the red carpet.
"I have the most extraordinary partner in life. I will always say that about him," the star said of Urban.
"I'm one of the very fortunate women in the world to have had a wonderful father, and I have a wonderful husband, and I'm just very -- I love to be able to say that, because to talk about the good men that have influenced me and affected me," she added.
Kidman and Urban are parents to two daughters, 10-year-old Sunday Rose and 8-year-old Faith Margaret. Kidman shared a rare photo with her girls Wednesday ahead of the premiere.
"I am nothing without the love of my family," she wrote, adding a heart emoji.
Big Little Lies Season 2 co-stars Reese Witherspoon, Shailene Woodley, Zoe Kravitz and Meryl Streep, and premieres June 9 on HBO. Witherspoon said during Wednesday's episode of Good Morning America she faces off with Streep's character in the new season.