In a multicenter European retrospective study including 162 patients with COVID-19 occurring in essential thrombocythemia (ET, n = 48), polycythemia vera (PV, n = 42), myelofibrosis (MF, n = 56), and prefibrotic myelofibrosis (pre-PMF, n = 16), 15 major thromboses (3 arterial and 12 venous) were registered in 14 patients, of whom all, but one, were receiving LMW-heparin prophylaxis. After adjustment for the competing risk of death, the cumulative incidence of arterial and venous thromboembolic events (VTE) reached 8.5% after 60 days follow-up. Of note, 8 of 12 VTE were seen in ET. Interestingly, at COVID-19 diagnosis, MPN patients had significantly lower platelet count (p < 0.0001) than in the pre-COVID last follow-up.This decline was remarkably higher in ET (-23.3%, p < 0.0001) than in PV (-16.4%, p = 0.1730) and was associated with higher mortality rate (p = 0.0010) for pneumonia. The effects of possible predictors of thrombosis, selected from those clinically relevant and statistically significant in univariate analysis, were examined in a multivariate model. Independent risk factors were transfer to ICU (SHR = 3.73, p = 0.029), neutrophil/lymphocyte ratio (SHR = 1.1, p = 0.001) and ET phenotype (SHR = 4.37, p = 0.006). The enhanced susceptibility to ET-associated VTE and the associated higher mortality for pneumonia may recognize a common biological plausibility and deserve to be delved to tailor new antithrombotic regimens including antiplatelet drugs.