Background: Cognitive impairment in obstructive sleep apnea (OSA) is a widely unrecognized entity. Earlier studies have shown variable results for the prevalence of this entity. Hence, this study was done to determine the prevalence of cognitive impairment in patients diagnosed to have OSA and its association with the severity of OSA. Materials and methods: All in-patients and outpatients who were diagnosed to have OSA by polysomnography in a tertiary care hospital in Kerala, India, were included in this study. Cognitive function was assessed by Montreal cognitive assessment (MoCA) and their scores were calculated. Daytime sleepiness was assessed by the Epworth sleepiness scale (ESS). Results: 96 OSA patients were included in the study. 19.8% of patients had mild OSA, 29.2% of patients had moderate OSA, and 51% of patients had severe OSA. The prevalence of cognitive impairment in OSA patients in this study was found to be 58.3% (95% confidence interval, 47.8–68.3%). There were 35 (62.5%) patients with cognitive impairment in the severe OSA group, while the moderate and mild OSA groups had 17 (30.4%) and 4 (7.1%) patients, respectively. There was a significant negative correlation between AHI and MoCA score, indicating that cognitive impairment is associated with the severity of OSA. Conclusion: The prevalence of cognitive impairment in our study was 58.3%, which was found to be more in those with severe OSA. It was also found that cognitive impairment is associated with the severity of OSA. Clinical implications: Cognitive impairment in OSA is associated with the severity of OSA and is a correctable condition that can improve the quality of life.