Hartnett, who has starred in 'Pearl Harbour', 'Black Hawk Down' and 'The Black Dahlia', will feature in both print and TV ads for the fragrance.
He is the centre of every angle in the ad campaign, which include shots that look like he is being photographed by paparazzi.
Giorgio Armani reportedly set out to find someone who would reflect the "bold allure of sexiness" that embodies this new fragrance.
The selection of Hartnett makes him the first male celebrity to represent Giorgio Armani Beauty.
Armani previously backed the women's version of the fragrance -- Emporio Armani Diamonds -- with a TV ad starring popstar Beyonce Knowles, in which she sang the song 'Diamonds are a Girl's Best Friend'.
Diamonds for Men will be available in September this year.
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