"They are very happily engaged," said Urban's publicist, Paul Freundlich. He declined to discuss details of the pair's wedding plans, and Kidman's publicist did not immediately return a call for comment Wednesday.
Kidman broke the news to People magazine Monday after hosting a weekend gala event in New York.
The announcement comes just weeks after Kidman told the 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 earlier this month for a story in its June edition.
Kidman and Cruise were married 10 years. They divorced in 2001 and have two adopted children.
For months, marriage rumors had swirled around Kidman and Urban. The couple, both 38, were first seen together in 2005, the AP reports.
Kidman won an Oscar for her role in the 2002 film "The Hours."
Urban, a singer and guitarist raised in Australia, won a Grammy in 2005 for best male country vocal performance.
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