Background: Sexual health is a critical component of overall wellness; however, only half of U.S. medical schools currently require formal instruction in sexuality. Aim: This study sought to quantify the sexual health knowledge of undergraduate medical students using a novel survey tool evaluating 6 domains: sexual function and dysfunction; fertility and reproduction; sexuality across the lifespan; sexual minority health; society, culture, and behavior; as well as safety and prevention. Methods: A novel 32-question survey tool was developed by subject matter experts from the University of Minnesota Program in Human Sexuality. Survey questions were derived from the 2012 and 2014 Summits on Medical School Education in Sexual Health as well as the Sexuality Information and Education Council of the United States Guidelines for Kindergarten through 12th Grade. The total knowledge score was calculated out of 30 points (excluding 2 terminology questions that were subjective). Medical students at 178 allopathic and osteopathic medical schools in the United States were invited to take the online survey. Outcomes: Students performed below a passing rate (70%) in 4 of the 6 knowledge categories and below a passing rate overall in the knowledge assessment. Results: Survey respondents (n = 1,014) scored an average of 66% correct (approximately 20/30). Overall, students scored lowest on questions regarding safety and prevention (x = 49%) and highest on questions regarding sexuality across the lifespan (x = 75%). Higher knowledge scores were associated with the following variables (P < .05): medical school year, race/ethnicity, sexual orientation, religious affiliation, future medical specialty choice, program type (MD/DO), and taking a human sexuality course in medical school. Clinical Implications: Medical students may be under-prepared to address essential sexual health issues in future clinical practice. Strengths & Limitations: To the knowledge of the authors this is the only contemporary study seeking to measure U.S. medical student sexual health knowledge. Limitations include sample population size and diversity as well as a non-validated survey tool. Conclusion: Significant advances must be made in undergraduate medical education in order to prepare future physicians to address critical issues such as sexually transmitted disease, family planning, and health disparities. Copyright (C) 2018, International Society for Sexual Medicine. Published by Elsevier Inc. All rights reserved.