Adenovirus may cause life-threatening infections. While outcomes in immunocompromised patients are well documented, data on critically ill, immunocompetent adults remain limited. To describe the clinical characteristics and outcomes of adenovirus infection in adult intensive care unit (ICU) patients and to identify factors associated with 30-day mortality. This retrospective observational study included adult patients (≥ 16 years) admitted to ICUs between January 2020 and May 2025 with PCR-confirmed adenovirus infection and compatible clinical syndromes, with a primary focus on describing patient characteristics rather than analyzing virological features of the infection. Data were collected from 24 centers in 11 countries. Demographics, comorbidities, clinical characteristics, laboratory findings, treatments, and outcomes were recorded. The primary outcome was 30-day all-cause in-hospital mortality. Variables associated with mortality in univariate analyses were entered into a stepwise multivariable logistic regression model. A total of 126 patients were included; 40 (31.7%) died within 30 days of PCR positivity. The mean age of the entire cohort of patients was 44.9 ± 20.4 years (median 48; 16-95), and 38.9% were female. Pneumonia was the most common presentation (108/126, 85.7%), followed by upper respiratory tract infection (18.3%), gastroenteritis (6.3%), conjunctivitis (4.8%), meningoencephalitis (2.4%), and cystitis (1.6%). Significant immunosuppression was present in 29.4%. In multivariable analysis, independent predictors of 30-day mortality included mean arterial pressure ≤ 65 mmHg, SOFA score ≥ 5, increasing Charlson Comorbidity Index, central venous catheter placement, platelet count <50,000/µL (borderline significance), procalcitonin ≥ 0.1 ng/mL, and significant immunosuppression. The final model had 84.9% accuracy, 72.5% sensitivity, and 90.7% specificity. A high percentage of ICU patients with PCR-confirmed adenovirus infection died within 30 days. Hemodynamic instability, organ failure severity, comorbidity burden, inflammatory response, thrombocytopenia, and immunosuppression are key determinants of poor outcome.