Southall was appointed as publishing director of Cosmopolitan in May 2007, having held previous publishing director positions at NatMag sister titles Company, She and Zest.
Southall will remain as Cosmopolitan's publisher until the end of the year when she leaves to set up her own media-related business project.
It is understood that NatMag will be announcing plans for her replacement in due course. A spokesperson confirmed the move but declined to comment further.
In September NatMags launched an , which targets 18- to 21-year-olds across 65 universities in the UK. The magazine is set to be distributed at the start of each term at least until the summer term 2011.
Cosmo in the UK is the third highest circulating paid-for magazine in the women's lifestyle sector, . The magazine suffered a 9% drop in circulation year on year to 401,750.
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