Objective To identify the clinical features and risk factors for mortality associated with severe influenza B pneumonia of adults admitted to respiratory intensive care unit (ICU). Methods Patients with confirmed influenza B infection and respiratory failure between February 2020 and February 2022 who were admitted to the ICU were sequentially included. Demographic features, clinical data, microbiological data, complications, and outcomes were collected. Univariate logistic regression analysis was performed to identify risk factors associated with hospital mortality. A comparison with severe influenza A pneumonia was made to explore the characteristics of influenza B virus-associated pneumonia. Results A total of 23 patients with influenza B pneumonia were included. The survival group included 18 patients and the death group included 5 patients, with an ICU mortality of 21.7%. The median age in the death group was 64 (64, 72.5) years, which was significantly older than the survival group, with a median age 59 (30.25, 64.25) years (P=0.030). Univariate logistic regression analysis indicated that SOFA score [odds ratio (OR) 1.307, 95% confidential interval (CI) 1.013 - 1.686, P=0.039], decreased hemoglobin (OR 0.845, 95%CI 0.715 - 0.997, P=0.046), and high blood urea nitrogen (BUN) (OR 1.432, 95%CI 1.044 - 1.963, P=0.026) were independent risk factors for hospital mortality. Compared with influenza A pneumonia, patients with severe influenza B pneumonia had more complications (60.0% vs. 87.0%, P=0.023). Conclusions The mortality of severe influenza B virus-associated pneumonia with was high. Increased SOFA score, anemia, and high BUN were risk factors for ICU mortality of severe influenza B infection in adults.