Objective: To determine individual- and country-level factors associated with self-reported changes in solo and partnered sexual behaviors in an international sample of adults during COVID-19. Methods: Data were from the International Sexual Health And REproductive Health during COVID-19 study (I-SHARE)—a cross-sectional, multi-country study (N = 26 countries) assessing adult (N = 19,654) sexual/reproductive health before and during the first wave of COVID-19. We examined self-reported changes (three-point scale: decreased, no change, increased) in solo masturbation, hugging/holding hands/cuddling with a partner, sex with a primary partner, sex with a casual partner, sexting with a partner, viewing sexually explicit media and partnered cybersex. Ordinal regression assessed the impact of individual (age, gender- and sexual-identity, romantic partnership status, employment and income stability, household change and content, mental well-being, changes in alcohol use, and changes in marijuana use) and country-level (e.g., Oxford Stringency Index, Human Development Index, and the Palma Ratio) factors on behavior change. Results: The most common behavior to increase was hugging, kissing, or cuddling with a partner (21.5%), and the most common behavior to decrease was sex with a main partner (36.7%). Household factors like job/income instability and having children over the age of 12 years were significantly associated with decreased affectionate and sexual partnered sexual behaviors; more frequent substance use was linked to significantly increased solo, partnered, and virtual sexual behaviors. Conclusions: Understanding changes in sexual behaviors—as well as the factors that make changes more or less likely among adults around the world—are important to ensure adequate sexual health support development for future public health emergencies.