Background: This study seeks to validate a radiological classification system of spontaneous upper urinary tract rupture (sUUTR) and to analyse its relationship with clinical, laboratory and radiological characteristics of sUUTR. Methods: We analysed data from 66 patients with a computerised tomography (CT)-proven sUUTR treated with ureteral or nephrostomy catheter positioning. Comorbidities were scored with the Charlson Comorbidity Index (CCI). All CT scans were reviewed by two experienced radiologists and one urologist, who classified sUUTR in (a) local spread, (b) free fluid and (c) urinoma. Interobserver agreement for radiological score was evaluated with the Intraclass Correlation Coefficient (ICC) and Cohen’s Kappa analyses. Descriptive statistics and logistic regression models verified the association between clinical variables and sUUTR severity. Results: The interobserver agreement for sUUTR classification was high among radiologists and between the radiologists and the urologist (all Kappa > 0.7), with an overall high interrater reliability (ICC 0.82). Local spread, free fluid and urinoma were found in 24 (36.4%), 39 (59.1%) and 3 (4.5%) cases, respectively. Patients with free fluid/urinoma had higher rate of CCI ≥ 1 than those with local spread (40.5% vs. 16.7%, p = 0.04). Intraoperative absence of urine extravasation was more frequently found in patients with local spread than those with free fluid/urinoma (66.7% vs. 28.6%, p < 0.01). Multivariable logistic regression analysis revealed that local spread (OR 4.5, p < 0.01) was associated with absence of contrast medium extravasation during pyelography, after accounting for stone size, fever and CCI. Conclusions: The analysed sUUTR classification score had good inter/intra-reader reliability among radiologists and urologists. Absence of urine extravasation was five times more frequent in patients with local spread, making conservative treatment feasible in these cases.