Introduction: There is a widespread prevalence of obesity and growing evidence suggests its possible adverse effect on cognitive functions. Hence, it is important to study the relationship between Body Mass Index (BMI) and Cognitive Functions. Aim: The aim of this study was to find out the association between BMI and cognitive functions in medical students. The present study also determined whether there are any gender based difference of association of BMI and cognitive functions. Materials and Methods: A cross-sectional study was done at Cognitive function test Laboratory of Dr. Vasantrao Pawar Medical College, Hospital and Research Centre, Nashik between July and December 2016. Study was done on randomly selected medical undergraduate students (50 male and 50 females) of 18-25 years age group. Height and weight were recorded and subjects were divided into 3 groups based on BMI (in kg/m2) as: Underweight (BMI <18.5 kg/m2), Normal weight (BMI 18.5-25 kg/m2) and Overweight (BMI >25 kg/m2). In all the above BMI groups, four domains of cognitive functions were assessed i.e., attention, perception, executive functions and working memory using Visual and Auditory Reaction Time (VRT and ART) and Go/No-Go VRT and ART; Fast Counting (FC); Eriksen Flanker Test (EFT) and Stroop Test (ST) and Visual Forward (VFDS) and Backward Digit Span (VBDS) test, respectively. The data was statistically analysed and different cognitive function tests were compared in various categories of BMI (viz., underweight, normal and overweight) using ANOVA and Tukey HSD tests. Gender based association of BMI and cognitive functions were also assessed. The data was analysed statistically using SPSS version 16.0 and p-value <0.05 was considered to be significant. Results: On comparing the cognitive functions with BMI, it was found that when whole study population was considered, the difference in time taken by subjects of different BMI categories to give response in VBDS test was found to be statistically significant (p=0.007). When the gender differences were analysed, it was found that in males, the time taken to give response to stimuli with interference (ST) in various BMI categories was statistically significant (p=0.004) and in females, the time taken to give response in VBDS test in various BMI categories was found to be statistically significant (p=0.024). The correlation between the reciprocal of time taken to give response in ST and VBDS and BMI was a weak positive correlation (0.3 < r < 0.5). Conclusion: A weak positive correlation was found between BMI and reciprocal of time taken to give response to stimulus with interference in males and BMI and reciprocal of time taken to give response in VBDS Test in Females.