Kidman hosted the 30th Anniversary gala for United Nations organization UNIFEM in New York City on Saturday, where she was accompanied by Urban.
The Hours star confirmed the engagement to People magazine, admitting Urban was not just her boyfriend saying, "He's actually my fiancé.
"I wouldn't be bringing my boyfriend (to the event)."
The confirmation of the engagement ends months of speculation about the Australian couple, who were first spotted together last July.
In November, Kidman was photographed wearing a ring on her wedding finger while walking arm-in-arm with Urban in Boston.
The actress, who has been in New York's Hamptons region shooting an untitled film for director Noah Baumbach, is a goodwill ambassador for UNIFEM, the United Nations Development Fund for Women.
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