Background Weight gain is a common challenge in correctional settings, where individuals often face environmental and behavioral constraints that increase their vulnerability to obesity. Incarcerated populations are exposed to limited opportunities for physical activity, poor nutritional options, high levels of psychological stress, and the potential side effects of medications, all of which may contribute to weight gain. Despite these risks, obesity among prisoners remains an under-researched area in Saudi Arabia. Methods A cross-sectional analytical study was conducted among 388 incarcerated individuals in correctional facilities in Makkah City between February and April 2025. Participants were selected through systematic random sampling, including only those incarcerated for six months or longer. Data were collected using a structured questionnaire covering sociodemographic characteristics, dietary patterns, physical activity, stress levels, and behavioral factors. Body mass index (BMI) was calculated using standardized measurements of height and weight. Statistical analyses, including chi-square tests and multivariable logistic regression, were performed using R (version 4.3.0) within the RStudio environment (R Foundation for Statistical Computing, Vienna, Austria). Results Of the 388 participants, 270 (69.6%) were classified as overweight or obese. Weight gain was significantly associated with older age (median 40 vs. 36 years, p < 0.001), limited access to physical activity (24/270 (8.9%) vs. 66/118 (56.0%), p < 0.001), frequent high-calorie food intake (136/270 (50.4%) vs. 27/118 (22.9%), p < 0.001), and elevated stress levels (median score 7 vs. 5, p < 0.001). Emotional eating was more common among individuals who gained weight (75/270 (27.8%) vs. 20/118 (17.0%), p = 0.02). In contrast, current smoking was inversely associated with weight gain (197/270 (73.0%) vs. 100/118 (84.7%); adjusted odds ratio (AOR) = 0.40, 95% CI: 0.18-0.84). Accessibility to exercise facilities demonstrated the strongest protective effect (24/270 (8.9%) vs. 66/118 (56.0%); AOR = 0.065, p < 0.001). Conclusion Obesity is highly prevalent among incarcerated individuals in Saudi Arabia and is influenced by a combination of environmental, psychological, and behavioral factors. Targeted interventions addressing physical activity, stress management, and nutrition are urgently needed in correctional settings.