Reducing hospitalization length of stay (LOS) for acute severe ulcerative colitis (ASUC) will reduce healthcare costs, mitigate hospitalization-associated risks (e.g., venous thromboembolism), and improve quality of life. A chart review was performed of all adult ASUC-related hospitalizations at University of California, San Francisco, from July 1, 2014, to December 31, 2017. Univariate and multivariate analyses were performed to identify factors associated with LOS < 7 days versus ≥ 7 days. A subgroup analysis was performed excluding patients who underwent colectomy during hospitalization. A total of 95 ASUC-related hospitalizations were identified. The initial univariable analysis identified the following factors associated with LOS ≥ 7 days (P < 0.05): higher maximum heart rate in the first 24 h, higher C-reactive protein, being biologic therapy naïve, and a later hospital day of biologic therapy initiation. On mixed model multivariable analysis, later hospital day of biologic initiation was associated with increased LOS ≥ 7 days (OR 3.1 95% CI 1.2-7.56, p = 0.012). We identified multiple predictors for longer hospital LOS, including factors related to disease severity (non-modifiable) and treatment (potentially modifiable). Importantly, this study identified biologic naïve treatment status and delayed inpatient biologic therapy initiation as predictors of longer LOS (≥ 7 days) in patients who did not ultimately require colectomy during their hospital stay. Potentially modifiable strategies to reduce LOS may include early communication and patient education about biologic therapy in both the inpatient and outpatient setting.