The efficacy of antithrombin (AT) administration in patients with septic shock and disseminated intravascular coagulation (DIC) was uncertain. This study aimed to investigate whether high-dose AT administration improves outcomes in patients with septic shock and DIC. This observational, prospective cohort study included consecutive adult septic shock patients with DIC who showed AT activity <70% between March 2016 and August 2018. The 28 day mortality of the patients treated with AT and without AT was evaluated by propensity score matching and inverse probability of treatment weighting. Among 142 patients with septic shock and DIC, 45 patients (31.7%) received AT supplementation and 97 did not. The 28 day mortality rate was lower in the AT group, but no statistically significant difference persisted after matching. Multivariable analysis showed that AT supplementation was independently associated with 28 day mortality (odds ratio [OR], 0.342; 95% CI [confidence interval], 0.133-0.876; P = 0.025); however, no such association was observed after matching (OR, 0.480; 95% CI, 0.177-1.301; P = 0.149). High-dose AT administration in septic shock patients with DIC showed the improvement in survival, but the improvement was not observed after matching. Further larger studies are needed to conclusively confirm these findings.