Background and Objectives: Overweight and obesity are growing public health challenges, particularly concerning young adults. University life presents a unique set of stressors that may influence weight management alongside sleep quality. In this cross-sectional study, we aimed to investigate the association between overweight or obesity, stress, and sleep quality in a large sample of Greek university students. Materials and Methods: The study recruited 2116 active students from across various Greek universities. Participants completed questionnaires on sociodemographics, academic performance, and physical activity levels using the International Physical Activity Questionnaire (IPAQ). Stress and sleep quality were assessed using the Perceived Stress Scale (PSS) and the Pittsburgh Sleep Quality Index (PSQI), respectively. Body weight and height were directly measured to calculate Body Mass Index (BMI). Results: Our analysis of 2116 Greek university students revealed significant associations between various factors and overweight/obesity. Compared to their rural counterparts, young adults in urban areas had an 88% higher prevalence of overweight/obesity (p = 0.0056). Regular smokers were twice as likely to be overweight or obese (p = 0.0012). Notably, those with low physical activity levels displayed a more than two-fold increased risk (p = 0.0008) compared to those with moderate or high activity levels. Similarly, students with moderate or high perceived stress levels had a more than two-fold prevalence of overweight/obesity compared to those with low stress (p = 0.0005). Inadequate sleep quality was also associated with an 86% higher risk of overweight/obesity (p = 0.0007). Interestingly, good academic performance showed a 57% greater prevalence of overweight/obesity compared to very good/excellent performance (p = 0.0103). Conclusions: Our findings reveal that perceived stress and poor sleep quality are significant risk factors for overweight and obesity in this young adult population.