IntroductionThe novel coronavirus disease 2019 (COVID-19) has quickly become a worldwide threat to human health and has significantly impacted people’s lives and changed their lifestyles and health behaviors. This study aims to assess lifestyle and health-related behaviors (LHBs) and associated factors among the general population in Lebanon during the COVID-19 pandemic.MethodsAn online cross-sectional study was conducted among 403 Lebanese adults. The study questionnaire was developed on Google Forms in English and Arabic. It included self-reported questions about sociodemographic characteristics, COVID-19, and perceived behavioral changes (smoking, alcohol consumption, sexual and hygiene behaviors, and intake of nutritional supplements and immunity-boosting foods). It also comprised three scales, i.e., the Lifestyle and Health Behaviors questionnaire (LHB-17), the WHO-5 Well-being Index, and the Fear of COVID-19 scale (FCV-19S).ResultsThe mean age of participants was 29.74 ± 10.81 years, with 51.2% females. Most participants reported that COVID-19 negatively affected their LHBs, mainly diet, sleep, and psychological well-being, while physical activity aspects remained unaffected. Female gender (Unstandardized Beta (ß) = − 2.336), living in Lebanon (ß = − 7.502), nargileh smoking (ß = − 3.433), high BMI (ß = − 0.263), and increased daily usage of electronic devices during the COVID-19 lockdown (ß = − 0.853) were significantly associated with lower LHB-17 scores, indicating worsened LHB. However, living in urban areas (ß = 2.464), employment status (ß = 1.920), good overall health (ß = 3.543), a higher quality of life (ß = 0.204), and unaffected physical (ß = 2.101) and mental (ß = 1.586) health during the COVID-19 lockdown were all significantly associated with higher LHB-17 scores, reflecting positive LHB.ConclusionLebanese adults reported several unfavorable lifestyle changes and psychological problems during the lockdown due to COVID-19, particularly affecting women, non-workers, waterpipe smokers, electronic device heavy users, people of lower socioeconomic status, and those with chronic diseases. Health promotion strategies are needed to assess negative changes both in physical and mental health and maintain as many positive health-related behaviors as possible among the Lebanese population.