Introduction: Chronic kidney disease (CKD) is a major global public health problem associated with increased risk of cardiovascular morbidity, premature mortality, and decreased quality of life. In Portugal, the PREVADIAB study showed a prevalence of CKD stages 3–5 of 6.1%. To overcome the limitations of the PREVADIAB study, the RENA study aimed to provide an estimate of the prevalence of CKD at a national level and to characterize CKD patients. Methods: This was a cross-sectional study including users of Primary Health Care Units aged 18 or more. After obtaining written informed consent, sociodemographic and clinical data were recorded through a structured questionnaire, anthropometric measurements were taken, and blood and urine samples were collected. All participants initially meeting the criteria for CKD were contacted at least 3 months after the initial assessment for confirmation of the analytical results. Results: A total of 3,135 individuals were included, 65.4% were female, and the mean age was 56.7 ± 15.9 years. The prevalence of hypertension, dyslipidemia, and diabetes was 38, 32, and 16%, respectively, and 31% were obese. After data adjustment by gender, age group, and geographical region, the global prevalence of CKD was 20.9% (95% CI: 6.5–35.3%), with no differences between genders and a significant increase with the advance of the age groups. Conclusion: Our study showed a CKD prevalence above the worldwide and Europe average. Despite the study limitations, it has become clear that it is urgent to identify CKD patients earlier and to develop awareness and educational programs to prevent CKD and its associated diseases.