Background Obstructive sleep apnoea (OSA) is one of the emerging non-traditional cardiovascular risk factors. Studying OSA may contribute towards a better understanding of current concepts of atherogenesis and in guiding therapy. Methods We conducted this cross-sectional study among 66 patients with acute coronary syndrome (ACS) in a tertiary care hospital from 1 January 2019 to 30 June 2020. We included patients of ST elevation myocardial infarction (STEMI)/ non-STEMI (on achieving Killip class I/II) and unstable angina and performed in-hospital overnight polysomnography (PSG) within 8 weeks of index event. Apnoea-hypoapnoea index (AHI) value 5-<15 was defined as mild OSA, AHI 15-<30 as moderate OSA and AHI >30 as severe OSA. We analysed data using Epi Info version 7.2.4 for Windows. Results The 66 patients had a mean (SD) age of 57.7 (11.1) years and 54 (81.8%) were men. Forty-three (65.1%) patients had STEMI, 19 (28.7%) had non-STEMI and 4 (6%) had unstable angina. On PSG, the prevalence of OSA (AHI>5) was 78.8% (95% CI 67.0-87.9). Of these, AHI >15 was significantly associated with diabetes, hypertension and different measures of obesity (p<0.05). Conclusions This study, conducted in a hill state of northern India, showed a high prevalence of OSA in patients with ACS. Obesity, diabetes mellitus and hypertension were significantly associated with severity of OSA (AHI>15).