The clinical profiles, presentation, and outcomes of patients with acute aortic dissections and associated periaortic hematomas on aortic imaging have not been described in a large cohort. This study sought to assess the prognostic implications of periaortic hematomas in patients with aortic dissections and to identify factors associated with in-hospital mortality in patients with periaortic hematomas. The study population was 971 patients with acute aortic dissections enrolled in the International Registry of Acute Aortic Dissection with available imaging data on presentation with the presence or absence of periaortic hematomas. Patients with periaortic hematomas (n = 227, 23.4%) were more likely to be women, to have a history of hypertension and atherosclerosis, and to present early to the hospital. At presentation, they had greater frequencies of shock, cardiac tamponade, coma, and/or altered consciousness. Clinical outcomes were significantly worse in patients with periaortic hematomas, including significantly greater mortality (33% vs 20.3%, p <0.001). A multivariate model demonstrated periaortic hematomas to be an independent predictor of mortality in patients with aortic dissections (odds ratio 1.71, 95% confidence interval 1.15 to 2.54, p = 0.007). In conclusion, this study provides insight into the profiles, presentation, and outcomes of patients with periaortic hematomas and acute aortic dissections. The early identification and aggressive management of patients with periaortic hematomas may potentially improve clinical outcomes.