"They are very happily engaged," said publicist Paul Freundlich. He declined to discuss details of their wedding plans, and Kidman's publicist did not immediately return a call for comment Wednesday.
The Oscar-winning actress broke the news to People magazine Monday after hosting a weekend gala event in New York.
"He's actually my fiance. I wouldn't be bringing my boyfriend," the magazine quoted her as saying in a story posted on its Web site.
The announcement comes just weeks after Kidman told Ladies' Home Journal that she still had feelings for her ex-husband, Tom Cruise.
"And I loved him. I still love him," she told the magazine for a story in its June issue.
Kidman and Cruise, who were married for 10 years, divorced in 2001. Cruise is engaged to Katie Holmes, who gave birth to their first child, a daughter, Suri, last month.
Urban and Kidman, both 38, were first seen together last year.
She won an Oscar for her role in the 2002 film "The Hours."
Urban, a singer and guitarist raised in Australia, won a Grammy in 2006 for best male country vocal performance for "You'll Think of Me."