BACKGROUND The Frailty Index (FI) is used to quantify and summarize vulnerability status in people. In Chile, no development and assessment of a FI have been explored. OBJECTIVE To develop and evaluate a FI using representative data from Chilean adults aged 40 years and older stratified by sex. METHODS Cross-sectional study. METHODS National representative data from the Chilean National Health Survey 2016-2017 (CNHS 2016-2017). METHODS 3,036 participants older than 40 years with complete data for all variables. METHODS A 49-item FI was developed and evaluated. This FI included deficits from comorbidities, functional limitations, mental health status, physical activity, anthropometry, medications, and falls. A score between 0 and 1 was calculated for each person. Descriptive statistics and linear regression models were employed to evaluate the FI's performance in the population. Comparative analyses were carried out to evaluate the FI score by age (1<60 and ≥ 60 years). RESULTS The mean FI score was 0.15 (SD:0.09), with a 99% upper limit of 0.46. Scores were greater in women than men (0.17 [SD:0.09]) vs. 0.12 [0.08]); in people older than 80 years (0.22 [0.11]), and in people with ≤8 years of education (0.18 [0.10]) compared with those with >12 years (0.12 [0.08]). The average age-related increase in the FI was 2.3%. When a cut-off point ≥ 0.25 was applied, the prevalence of frail individuals was 11.8% (95% CI: 10.0 to 13.8) in the general population. The prevalence was higher in women 15.9% [95% CI: 13.3 to 18.9] than men 7.4% [95% CI: 5.3 to 10.1]. In a comparative analysis by age, higher FI mean scores and prevalence of frail were observed in people ≥ 60 than younger than 60. CONCLUSIONS The mean FI score and frailty prevalence were higher in women than men, in people with fewer years of formal education, and incremented markedly with age. This FI can be used for early detection of frailty status focusing on women and middle-aged people as a strategy to delay or prevent frailty-related consequences.