Background: The word ‘sex’ in India is taboo and is not discussed openly. Sex attitude refers to one’s attitude towards sexuality or different sexual behaviours. Poor sex knowledge causes many problems. A liberal attitude without adequate knowledge is harmful. Aim: To explore and study sexual knowledge, attitudes, behaviours and the sources of influence and to examine the relationship between sexual knowledge, attitude and behaviours in undergraduate medical students. Material and Methods: A cross-sectional study was carried out to assess sex knowledge and attitude among 506 medical undergraduate students. A Google document in the form of a structured questionnaire containing three important parts was used for the assessment: (a) demographic details of students; (b) sex knowledge and attitude questionnaire II (SKAQ II); and (c) Sexual Behaviour and Sources of Influence (SBSI) scale. Data was interpreted using mean, unpaired ‘t’ test and chi-square test. Results: Overall, participants showed a conservative attitude towards sex. As the academic year of participants progressed, sex knowledge increased. A statistically significant positive correlation was observed between sex knowledge and attitude scores. Sexual behaviours through media and with self or others were found to be low. The Internet was found to be the major source for gathering information and was considered the most reliable source. Conclusion: Participants were deficient in sex knowledge and had a conservative attitude in certain areas. Females, higher education levels and urban area participants had higher sex knowledge and liberal attitudes. There are areas of sex knowledge and attitude that need improvement by proper sex education.