The study analyzed the association between hematocrit (HCT) and 3-month adverse outcome in 1896 South Korean patients with acute ischemic stroke (AIS), including 1163 men (61.34%) and 733 women (38.66%). An unfavorable outcome (modified Rankin score ≥ 3) had a probability of 28.53%, higher in women (35.20%) than in men (24.33%). In the overall population, there was a non-linear relationship with HCT below 34.4%, where a 1% increase in HCT reduced the risk by 15% (OR = 0.85, 95% CI 0.78–0.92, P = 0.0001). In men, a similar non-linear relationship was shown for HCT below 36.4% (95% CI 32.2–38.6%), with a 19% reduction in risk per 1% increase (OR = 0.81, 95% CI 0.74–0.88, P < 0.0001). No significant association was found in women. The study identified a sex-specific non-linear relationship between HCT and outcome after AIS.