Measuring vulnerability and identifying determinants of vulnerability are key to designing interventions for marginalized groups like sex workers. The current study introduces a new approach of measuring vulnerability among female sex workers (FSWs) by adopting a multidimensional poverty measurement framework. A multidimensional vulnerability index was created from four dimensions and 16 indicators using a dual cut-off approach. The study found that 55% of FSWs were multidimensionally vulnerable with 48% of intensity in vulnerability. The overall value of multidimensional vulnerability index was 0.265. FSWs in Maharashtra were most vulnerable (82%). Lack of financial security contributed mostly to FSWs’ vulnerability. Further, compared to less vulnerable FSWs, multidimensionally vulnerable ones were more to engage in behaviors that put them at risk such as inconsistent use of condoms with clients, alcohol consumption, engaging in anal sex with clients and experiencing sexually transmitted infections. Findings suggest that structural, social and financial vulnerabilities of FSWs need to be addressed concurrently.