Background: Social distancing in the wake of the coronavirus disease 2019 (COVID-19) pandemic may affect the sexual behavior of men who have sex with men (MSM). In early March 2020, Israel imposed travel restrictions and limited social contacts to household members only. The effects of these restrictions on the sexual behavior and mental health of MSM are unknown. Aim: To assess sexual behaviors and mental health of Israeli MSM during social distancing and to compare sexual behaviors before and during social distancing, due to the COVID-19 pandemic. Methods: Data were collected through anonymous web-based questionnaires in a popular geospatial application used by MSM between March and April 2020 during the social-distancing period. Outcomes: The dependent variable was casual sex, in violation of social-distancing regulations. Independent variables were demographic characteristics, sexual behaviors before and during social-distancing restrictions, and mental health. Results: Of the 2,562 participants, 1,012 (39.5%) continued to meet new casual sex partners during this period. Being of a younger age, single, and with higher levels of mental distress predicted engagement in casual sex during the social-distancing period. MSM reduced their sexual risk and limited sexual repertoire-in particular, kissing with their sexual partners. Participants also spent more time in dating applications than in the preesocial-distancing period and increased their use of sex phone, webcams, and porn consumption. They perceived the threat of severe acute respiratory syndrome coronavirus to be greater than that of HIV: only 3.2% could imagine themselves having sex with a partner who is infected with SARS-CoV-2 compared with 30.1% in case of HIV, P < .01. Clinical Implications: MSM reduced their risk behaviors during social distancing because of the threat of COVID-19. Casual sex during social distancing was associated with negative feelings of mental distress. Future public health response in the future waves of COVID-19 morbidity should strike a balance between containment measures and the need for social distancing with its possible mental and social burdens. Strengths and Limitations: This is the first study in Israel and one of the few in the world to examine sexual behaviors among MSM during the COVID-19 social distancing period. It involved a relatively large sample, through convenience sampling, which limits causality. Findings should be interpreted cautiously, specifically because COVID-19erelated behaviors and circumstances may change rapidly. Conclusion: The negative feelings of distress due to social distancing should be considered as a potential barrier to adherence among vulnerable populations, such as MSM. Future public health response should strike a balance between containment measures and its possible mental, social, and financial burdens.