The X Factor judge topped the lads' magazine poll ahead of her predecessor Cheryl Cole, who won the title in 2009 and 2010.
Rihanna is a non-mover from last year's vote at three, while glamour model Rosie Jones and newcomer Georgia Salpa round out the Top Five.
Speaking about her victory, Contostavlos declared: "It's a true honour and definitely a lovely confidence boost. I'm proud of me and I am who I am. I know that I'm marmite and I wouldn't wanna be anything less or anything more, I'm just myself."
The 'Young' singer, who hit the headlines recently for starring in a sex tape, added: "I believe in embracing being sexy, but I'm not one to agree with stripping off for the cameras.
"I like to be more subtle than that as these shots show. This is an award that will stay on the mantelpiece probably for the rest of my life."
> Georgia Salpa in FHM's 100 Sexiest Women - pictures
High-charting new arrivals in the FHM Top 100 include Pippa Middleton at 12, Mollie King of The Saturdays at 14, Made in Chelsea's Millie Mackintosh at 15 and New Girl star Zooey Deschanel at 17.
Last year's winner Rosie Huntington-Whiteley crashes to 18, while Contostavlos's X Factor protégée Jade Thirlwall of Little Mix and 44-year-old presenter Kate Garraway are among the more notable inclusions, finishing at 49 and 97 respectively.
The Top 10 of FHM's '100 Sexiest Women In The World' in full:
1. Tulisa Contostavlos
2. Cheryl Cole
4. Rosie Jones
5. Georgia Salpa
6. Katy Perry
7. Megan Fox
8. Keeley Hazell
9. Mila Kunis
10. Emily Atack
Watch the video for Tulisa's 'Young' below: