Background Obstructive sleep apnea ( OSA ) is present in 60% to 70% of stroke patients. Cerebral vasoreactivity in patients with stroke and OSA has not been well studied and could identify a new pathophysiologic mechanism with potential therapeutic intervention. We aimed to determine whether risk categories for OSA are associated with cerebral vasoreactivity in stroke patients. Methods and Results In this cross‐sectional study of a cohort of patients with stroke, we used clinical questionnaires (Sleep Obstructive Apnea Score Optimized for Stroke [ SOS ] and snoring, tiredness, observed, pressure, bmi, age, neck, gender [ STOP ‐ BANG ] scores) to assess the risk of OSA and transcranial Doppler to assess cerebral vasoreactivity (breath‐holding index and visual evoked flow velocity response). Of the 99 patients included, 77 (78%) had medium or high risk of OSA and 80 performed transcranial Doppler. Mean breath‐holding index was 0.52±0.37, and median visual evoked flow velocity response was 10.8% (interquartile range: 8.8–14.5); 54 of 78 (69%) showed impaired anterior circulation vasoreactivity (breath‐holding index <0.69) and 53 of 71 (75%) showed impaired posterior circulation vasoreactivity (visual evoked flow velocity response ≤14.0%). There was a significant negative correlation between the risk of OSA calculated by STOP ‐ BANG and the breath‐holding index ( r S =−0.284, P =0.012). The following variables were associated with low anterior circulation vasoreactivity: dyslipidemia (odds ratio: 4.7; 95% CI , 1.5–14.2) and STOP ‐ BANG score (odds ratio: 1.7 per 1‐point increase; 95% CI , 1.1–1.5). Conclusions A high risk of OSA and impaired vasoreactivity exists in the population that has had stroke. Dyslipidemia and STOP ‐ BANG sleep apnea risk categories were independently associated with impaired anterior circulation vasoreactivity.