Background: Cardiovascular diseases (CVDs) are the leading cause of morbidity and mortality worldwide, with metabolic syndrome and its risk factors contributing substantially to cases in Latin America. In southern Chile, obesity, dyslipidemia, and sedentary behavior are highly prevalent, yet comprehensive baseline data on these factors are scarce. Establishing regional cardiometabolic profiles is crucial to inform prevention strategies. Objective: To describe the RIO-Study protocol and characterize the baseline cardiometabolic profile of adults from Valdivia, southern Chile. Methods: The RIO-Study is a randomized, double-blind, placebo-controlled, crossover clinical trial evaluating the effects of nutritional doses of seaweed-derived omega-3 fatty acids on lipid metabolism, inflammation, and molecular lipid regulators in adults with overweight/obesity. The protocol includes a standardized high-fat breakfast challenge and repeated postprandial blood sampling to assess dynamic lipid responses. Screening procedures comprised blood pressure measurement, fasting blood sampling, body composition by bioelectrical impedance, and health and lifestyle questionnaires. Results: Among screened participants, 91% presented overweight/obesity and 55% presented central adiposity, exceeding regional cardiometabolic risk thresholds (waist circumference ≥ 90 cm in men, ≥80 cm in women). Men exhibited higher waist circumference (100 ± 10.8 vs. 91.6 ± 11.9 cm), waist-to-hip ratio (0.99 ± 0.08 vs. 0.92 ± 0.07), systolic blood pressure (130 ± 12.0 vs. 122 ± 13.4 mmHg), triglycerides (168 ± 84.7 vs. 122 ± 64.9 mg/dL), VLDL-C (33.7 ± 17.2 vs. 24.4 ± 13.0 mg/dL), and sedentary time (8.1 ± 2.3 vs. 6.8 ± 2.3 h/day). Women had greater total body fat (39.7 ± 4.75% vs. 31.1 ± 5.30%), higher HDL-C (56.6 ± 13.3 vs. 46.9 ± 9.39 mg/dL), and more often had normal weight (13% vs. 0%). Conclusions: The RIO-Study provides novel insights into cardiometabolic risk and will elucidate the effects of nutritional omega-3 supplementation in a high-risk Chilean population.