Introduction: Patients with chronic airway disease may present features of both asthma and COPD, commonly referred to as asthma–COPD overlap syndrome (ACOS). Recommendations on their diagnosis are diffuse and inconsistent. This survey aimed to identify consensus on criteria for diagnosing ACOS. Methods: A Belgian expert panel developed a survey on ACOS diagnosis, which was completed by 87 pulmonologists. Answers chosen by ≥70% of survey respondents were considered as useful criteria for ACOS diagnosis. The two most frequently selected answers were considered as major criteria, others as minor criteria. The expert panel proposed a minimal requirement of two major criteria and one minor criterion for ACOS diagnosis. Respondents were also asked which criteria are important for considering inhaled corticosteroids prescription in a COPD patient. Results: To diagnose ACOS in COPD patients, major criteria were “high degree of variability in airway obstruction over time (change in forced expiratory volume in 1 second ≥400 mL)” and “high degree of response to bronchodilators (>200 mL and ≥12% predicted above baseline)”. Minor criteria were “personal/family history of atopy and/or IgE sensitivity to ≥1 airborne allergen”, “elevated blood/sputum eosinophil levels and/or increased fractional exhaled nitric oxide”, “diagnosis of asthma <40 years of age”; “symptom variability”, and “age (in favor of asthma)”. To diagnose ACOS in asthma patients, major criteria were “persistence of airflow obstruction over time (forced expiratory volume in 1 second/forced vital capacity ratio <0.7)” and “exposure to noxious particles/gases, with ≥10 pack-years for (ex-)smokers”; minor criteria were “lack of response on acute bronchodilator test”; “reduced diffusion capacity”; “limited variability in airway obstruction”; “age >40 years”; “emphysema on chest computed tomography scan”. Conclusion: Specific criteria were identified that may guide physicians to a more uniform diagnostic approach for ACOS in COPD or asthma patients. These criteria are largely similar to those used to prescribe inhaled corticosteroids in COPD.