Objective To evaluate the prognostic impact of tumour size on survival outcomes in upper urinary tract urothelial carcinoma (UTUC) treated with radical nephroureterectomy (RNU). Patients and Methods Data on 795 patients treated with RNU for UTUC from seven centres were retrospectively analysed with focus on tumour size. Clinicopathological features and relevant prognostic factors were compared between patients with tumours <= 3.0 cm and those with tumours >3.0 cm in size. The primary endpoints were cancer-specific survival (CSS), disease recurrence-free survival (RFS) and overall survival (OS). Results At a median follow-up of 32 months, 313 (39.4%) patients died from UTUC, 321 (40.4%) developed cancer recurrence, and 359 (45.1%) died from all causes. Tumour size >3.0 cm was associated with unfavourable clinicopathlogical features. Kaplan-Meier analysis showed that tumour size was significantly correlated with worse CSS, RFS and OS (all P < 0.001). Multivariate analysis showed that tumour size was an independent predictor of CSS (hazard ratio [HR] 2.296; P < 0.001), RFS (HR 2.193; P < 0.001) and OS (HR 2.417; P < 0.001). Conclusions Tumour size >3.0 cm was a significant predictor of CSS, RFS and OS after RNU for patients with UTUC. Further studies are warranted before tumour size is included in risk prediction tools.