Purpose Chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) are associated with polycythemia. However, there still remain unanswered questions about the relationship between overlap syndrome (OVS), where OSA and COPD coexist, and polycythemia. Here, we aimed to establish the prevalence of polycythemia in OVS patients and to explore the impact of OSA on polycythemia. Patients and Methods Patients with COPD underwent overnight polysomnography (PSG), pulmonary function tests, echocardiography, and complete blood counts. All patients were ethnic Han Chinese and free of prolonged oral corticosteroid use, hematological system disease, severe systemic disease, and other sleep-disordered breathing. OVS was defined as COPD patients with an apnea–hypopnea index ≥15 events/h, and polycythemia was defined as an Hb >165 g/L in men and >160 g/L in women. Results Eight-hundred and eighty-six patients with COPD were included in the analysis. The prevalence of polycythemia was significantly higher in OVS patients than COPD-alone patients (6.4% vs 2.9%, p < 0.05). The prevalence of polycythemia increased with OSA severity ( χ 2 = 7.885, p = 0.007), but not in GOLD grade 3–4 COPD patients ( χ 2 = 0.190, p = 0.663). After adjusting for confounders, percentage of total sleep time with SaO 2 <90% (TS 90 ) remained independently associated with an increased odds of polycythemia (OR 1.030, 95% CI 1.015–1.046) and, with an increase in TS 90 , the hemoglobin increased, especially in GOLD grade 1–2 patients ( p < 0.05). Conclusion Patients with OVS have a higher prevalence of polycythemia than those with COPD alone, and TS 90 is an independent factor for polycythemia, especially in GOLD1-2 COPD patients.