Undernutrition is one of the major problems especially among the tribal adolescent girls. Thus, the objective of the present study was to evaluate the prevalence of thinness and its association with socio-economic and demographic aspects among adolescent girls. This study was conducted among 418 tribal adolescent girls, aged 10-17 years of Kharagpur Development Block II, West Bengal, India. Assessment of nutritional status was measured by thinness (Low BMI for age) following standard cut offs as proposed by Cole and co-workers. Logistic regression was applied to evaluate the dependence of thinness on various socio-economic factors of the participants. The overall prevalence of thinness was 43.5%. When age specific prevalence are taken into account, the highest (67.5%) prevalence was found among the participants of the age group of 10 years, followed by age group 12 (54.7%) and in case of 17 years (49.0%). Prevalence of thinness was highly (p < 0.001) dependent on mothers’ education (Wald = 10.3481) and physical activity (Wald = 6.8404) of the participants. Higher the mother’s education, lower the prevalence of thinness. The prevalence of thinness was very high. Future nutritional and health intervention programmes should focus on these factors.