Background The coronavirus (COVID-19) pandemic has created an unprecedented problem in people's lives around the world. Lockdown measures altered the routine lifestyle aspects of people including diet, exercise, sleep, stress, smoking, job status, recreation, and application of modern technologies. Understanding the lifestyle profile of individuals could help in designing effective interventions to minimize the risk factors of COVID-19-related health problems. Objectives The aim of this research is to investigate the lifestyle changes among adults living in Saudi Arabia (SA) during the COVID-19 pandemic. Methods A cross-sectional survey study was done to investigate the lifestyle changes during the COVID-19 pandemic in Saudi Arabia from August to September 2020. A pre-designed questionnaire was used for data collection and distributed online through social media. The questionnaire included items about sleep patterns, dietary habits, physical activity, employment status, recreation activities, use of social media, and screen time before and during the pandemic. Results A total of 338 adults with a median age of 40 years participated in the study. During the pandemic, employment, smoking decreased significantly (53.3 vs. 55.6%, p<0.001 & 15.7% vs. 18.3%, p=0.049) with significant increases in daily intake of fruits or vegetables (47% to 60.2%, p<0.001), caffeinated beverages (seven or more times caffeine, 3% vs. 0.9%, p<0.001), and water (more than eight water cups daily, 18.4 vs. 11.9, p<0.001). Overweight group increased significantly from 28.5% to 32% (p=0.009). Sleeping more than nine hours increased significantly from 8.3% to 21.8% (p<0.001) with increasing sleeping aids from 11.6% to 15.7% (p<0.001). Both the screen and the social media times increased significantly for six or more hours daily (14.8% vs. 35.3% and 9.5% vs. 28.2% respectively, p=<0.001 for both). There were significant decreases in socialization (91.4% vs. 37.8%) and fast food (71.98% vs. 47.04%), and increases in physically inactivity (19% vs. 5.3%) and stress (90.8% vs. 85.2%) (p<0.001 for all). Most participants perceived stress during the pandemic (307, 90.8%). Conclusion There were remarkable behavioral changes in all aspects of the lifestyle of the participants living in SA during the COVID-19 pandemic with some positive effects on smoking and dietary habits. However, negative changes included unemployment, physical inactivity, sleep disturbances, social isolation, and excessive weight gain. There is a need to study the possible consequences of such changes on the future population health in SA.