The travel-size edition will have a print run of 50,000 and will go on sale in WH Smith Travel stores and other outlets in London to target commuters and travellers. It will run alongside a regular-sized edition of Cosmopolitan and have the same content.
The trial starts with the October issue, which hits newsstands on 18 September and could be extended through January and February.
The move will be seen as a defensive one. Conde Nast's Glamour has recently extended its sales lead over Cosmopolitan by more than 75,000 copies and tops the women's monthly glossy sector.
However, Duncan Edwards, NatMags' managing director, denied this: "Cosmopolitan in Greece, France and Spain is already available in both formats. This pre-dates the launch of Glamour."
Glamour is expected to extend its circulation lead, with figures due later this week.
The format change comes just two months after NatMags cut the cost of its July issue by £1 to £1.80 outside the south-east to match Glamour.
Conde Nast accused NatMags of a strategic about-turn. Nicholas Coleridge, the managing director of Conde Nast, said: "After everything NatMags has said about Glamour, this is the biggest strategic U-turn since Margaret Thatcher junked the Poll Tax. The words 'desperate' and 'knee-jerk' rather spring to mind."