Over the past few decades, people in Saudi Arabia have become less inclined to adopt active lifestyles and healthy eating habits due to the increasing use of digital technologies such as social media. The objective of this online-based cross-sectional study was to assess the role of social media food advertisements and physical activity on eating behaviors among the general population in Saudi Arabia (n = 471). Data were collected through a structured questionnaire, which consisted of four parts: (i) sociodemographic information, (ii) attitude towards social media, (iii) eating behaviors-related information, and (iv) exposure to and engagement with social media advertisements. The study’s outcome variable, eating behaviors (healthy vs. unhealthy), was assessed using the following question: “Are you on a healthy diet (such as a balanced diet, keto, or low carb)?” A multiple binary logistic regression analysis was performed to investigate the factors that influence unhealthy eating behaviors. Approximately 79.6% of the participants had unhealthy eating behaviors. Participants who were not involved in daily physical activity were more likely to have unhealthy eating behaviors compared to their counterparts (adjusted odds ratio, AOR = 4.86). Participants who watched food ads on social media channels 1–3 times a week (AOR = 2.58) or daily (AOR = 3.49) were more likely to have unhealthy eating behaviors than their counterparts. Participants whose appetite to try foods increases always (AOR = 1.42) or usually (AOR = 2.88) after viewing ads on social media were more likely to have unhealthy eating behaviors. These findings suggest that policymakers should take immediate action to regulate food advertising policy to promote a healthy food environment across the country. Saudis should be encouraged to engage in more physical activity, which could support the maintenance of healthy eating patterns and lifestyles.