The change, which is understood to be the first by a magazine publisher in the UK, means that Cosmo Girl will publish 17 issues in the space of a year rather than the current 12.
According to NatMags, the impetus for the move was demand from readers for more of the magazine.
The cover price of £2 and the editorial content will remain unchanged. Covermounts will be offered with every issue.
Jan Adcock, Cosmopolitan group publishing director, said: "By increasing frequency against a growing readership, we are offering advertisers more opportunities to reach a lucrative, product-hungry market. Shorter lead times will enable us to be more accommodating to time-sensitive promotions."
NatMags will be hoping that the change in frequency adds impetus to its sales as Cosmo Girl chases rivals Sugar and Bliss.
The January-June 2004 ABC figures show Cosmo Girl selling 200,168 copies, with Hatchette Filipacchi Sugar leading the market on 295,009, ahead of Emap's Bliss with 257,162.
Cosmo Girl has been one of the bright spots of the market since it launched, steadily putting on sales as it crashed through the 200,000 barrier in the last set of circulation figures.
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