ObjectiveIt was necessary to understand the determinants of severe COVID-19 in order to deliver targeted healthcare services to prevent further complications and mortality. Identifying the factors associated with severe COVID-19 in Addis Ababa, Ethiopia, is the aim of this study.DesignA case–control study was conducted from October 2021 to March 2022.SettingThe study was conducted at three public COVID-19 treatment centres including Ekka Kotebe General, St. Peter Hospital and St. Paul’s Hospital.ParticipantsThe study participants were COVID-19 patients admitted to three COVID-19 treatment centres. Cases were patients admitted with severe COVID-19, and controls were patients with mild or moderate COVID-19. A total of 306 patients (153 cases and 153 controls) selected by simple random sampling technique participated in this study.Outcome measures and analysisData were collected by a face-to-face or telephone interviewer using a structured questionnaire. COVID-19 admission category, clinical and biomedical characteristics and comorbidity-related data were extracted from the participant’s medical record. Multivariable binary logistic regression analysis was used to identify predictors of COVID-19 severity.ResultsThe odds of being old were 4.54 times higher among severe COVID-19 cases (adjusted odds ratio (AOR)=4.54, 95% CI=2.499 to 8.24), the odds of being male were 2.72 times higher among severe COVID-19 cases (AOR=2.72, 95% CI=1.46 to 5.057), being vaccinated for COVID-19 decreases the severity by 55.1% (AOR=0.449, 95% CI=0.251 to 0.801), having good knowledge about COVID-19 decreases by 65% (AOR=0.35%, 95% CI=0.195 to 0.63) among patients with severe COVID-19, the odds of being diabetic were 10.2 times higher among severe COVID-19 cases (AOR=10.2, 95% CI=4.596 to 22.61) and the odds of being hypertensive were 2.3 times higher among severe COVID-19 cases (AOR=2.26, 95% CI=1.092 to 4.685).ConclusionMale, older age, diabetes or hypertension comorbidity, COVID-19 vaccination and having inadequate knowledge about COVID-19 were determinant factors of severe COVID-19.