Lifestyle changes are causing an exponential increase in the prevalence of obesity and metabolic syndrome (MetS) worldwide. The most frequent complications of these are the development of diabetes (T2D) and cardiovascular disease (CVD). Accurate tools are needed to classify the cardiovascular risk (CVR) in the MetS population. In recent years, numerous biomarkers of bone metabolism have been associated with CVR. The aim of this study was to determine the levels of undercarboxylated osteocalcin (ucOC) in a cohort of patients with MetS and to analyse its association with MetS parameters and CVR as well as with T2D prevalence. A longitudinal study was conducted in which a MetS population was followed for one year. Weight change, adherence to the Mediterranean diet (MedDiet), ucOC levels, MetS parameters and CVR were analysed and CVR was calculated using different scores. Our results showed a decrease of CVR associated with a better adherence to the MetDiet resulting in higher HDL-C and ucOC levels though the improvement of MetS risk factors. This bone protein appeared as a potential biomarker to classify CVR in the MetS population, especially for MetS patients without prevalent T2D. Furthermore, ucOC serum levels could be good predictors of T2D prevalence.