There is little information on the dietary modulation of thrombosis-related risk factors such as platelet count. We aimed to assess the effects of Mediterranean diet (MedDiet) on platelet count and related outcomes in an older population at high cardiovascular risk. In participants of the PREDIMED (PREvención con DIeta MEDiterránea) study, we assessed whether an intervention with a MedDiet enriched with extra-virgin olive oil or nuts, relative to a low-fat control diet, modulated platelet count (n = 4189), the risk of developing thrombocytosis and thrombocytopenia (n = 3086), and the association between these alterations and all-cause mortality (median follow-up time: 3.0 years). Although platelet count increased over time (+0.98·10⁹ units/L·year [95% confidence interval: 0.12; 1.84]), MedDiet interventions moderated this increase, particularly in individuals with near-high baseline count (both MedDiets combined: −3.20·10⁹ units/L·year [−5.81; −0.59]). Thrombocytopenia incidence was lower in the MedDiet interventions (incidence rates: 2.23% in control diet, 0.91% in MedDiets combined; hazard ratio: 0.44 [0.23; 0.83]). Finally, thrombocytopenia was associated with a higher risk of all-cause mortality (hazard ratio: 4.71 [2.69; 8.24]), but this relationship was attenuated in those allocated to MedDiet (p-interaction = 0.018). In brief, MedDiet maintained platelet counts within a healthy range and attenuated platelet-related mortality in older adults at high cardiovascular risk.