Background/Objectives: Weight management is crucial for preventing and managing diabetes. This study aimed to evaluate the knowledge, attitude, and practice (KAP) of people with Type 2 Diabetes Mellitus (T2DM) regarding weight management in Saudi Arabia. Methods: This cross-sectional survey of patients with T2DM in Saudi Arabia was conducted from October 2024 to February 2025. Participants completed a self-administered online questionnaire, which was validated and adapted from a previous study. Results: Among 385 individuals, 60.0% were females, and 34.0% were between 18 and 25 years old. The average scores for knowledge, attitude, and practice were 8.7 ± 3.6 (possible range: 0–13), 28.5 ± 4.4 (possible range: 10–50), and 28.3 ± 4.0 (possible range: 9–45), respectively. Pearson’s correlation analysis shows no significant correlation between knowledge and attitude (r = −0.016, p = 0.750). However, a weak yet statistically significant positive correlation was found between knowledge and practice (r = 0.155, p = 0.002). Additionally, a moderate negative correlation was observed between attitude and practice (r = −0.353, p = 0.001). Conclusions: The participants exhibited moderate knowledge, negative attitudes, and moderate practices regarding weight management. These findings provide crucial guidance for developing healthcare strategies to improve the KAP of diabetic patients in weight management.