The title, owned by The National Magazine Company, is fighting for readers in the ultra-competitive women's monthly sector, led by Conde Nast's Glamour.
The London poster campaign, which breaks on Monday, is the first burst in a planned three-year period of brand-building activity. It uses several executions from Clemmow Hornby Inge using the line "Bring out the Cosmo in you", in an attempt to communicate how it tries to inspire young women.
Jan Adcock, the group publishing director at NatMags, said: "With 33 years of brand saliency, it is appropriate that we should invest in the brand to ensure continued success."
Cosmopolitan, which has launched a handbag-sized edition to compete with Glamour, currently has its highest circulation in 24 years, at 478,394.
However, Glamour is growing at a faster rate than its rival and now has a circulation figure of 620,391.