The 35-year-old actress is currently "drawn" to "big, baggy" garments that resemble "chimney-sweeper trousers", but the blonde-haired beauty has admitted she has no idea where her interest in the androgynous style has stemmed from.
Speaking to WWD about her fashion favourites, Sienna who has four-year-old daughter Marlowe with her former partner Tom Sturridge said: "I don't really know where it came from, but I'm quite into quite masculine workman's clothes. I keep getting drawn to big, baggy, sort of chimney-sweeper trousers and tweeds. High-waisted, bunched-up things and slightly odd shoes."
However, the 'American Sniper' star has revealed her wardrobe has changed from being "quite flirty and sexy" in the past to now adorning "do-not-fancy-me" clothes.
Speaking about her style evolution, the New York-born star who has starred in campaigns for designer brand Burberry said: "I used to be quite flirty and sexy, and now it's just like, 'Do-not-fancy-me' clothes."
But Sienna prefers to look "unattractive" and have as little flesh on show as possible.
She added: "The more unattractive I can make myself, the less flesh I can possibly show, the better."
Meanwhile, Sienna who has recently been announced as the face of Swedish company Lindex has revealed her ultimate favourite fashion label is Chloé, although she may "hate" an item until she sees the label and then she becomes "obsessed" with it within a matter of moments.
The 'The Lost City of Z' star who portrays Nina Fawcett in the drama film said: "I can hate it [a product] and see it's got that Chloé label and be obsessed with it in a second."