Increased malignancy after kidney transplantation (KT) is by far the most troublesome issue. Among these malignancies, urothelial carcinoma (UC) incidence is uniquely high in Taiwan. We want to know whether routine sonography to detect native hydronephrosis is associated with the development of de novo urinary bladder urothelial carcinoma (UBUC) in post-KT recipients. From 2003 to 2018, we retrospectively analyzed 1005 KT patients, 58 of whom were subsequently diagnosed with UBUC. The association between new native hydronephrosis and post-KT UBUC was analyzed with univariate and multivariate logistic regression analyses and a Kaplan–Meier plot. We excluded cases of people who had upper urinary tract urothelial carcinoma (UTUC) and were diagnosed prior to UBUC. There were 612 males (60.9%) and 393 females (39.1%), with a mean age of 48.2 ± 12.0 years old at KT. The mean follow-up period was 118.6 ± 70.2 months, and the diagnosis of UBUC from KT to UBUC was 7.0 ± 5.1 years. New native kidney hydronephrosis occurred more frequently in the UBUC group (56.4% versus 6.4%,p< 0.001) than the non-UBUC group. Multivariate analysis disclosed that native hydronephrosis is the only statistically significant factor for UBUC, with an odds ratio of 16.03 (95% CI, 8.66–29.68;p< 0.001). UBUC in post-KT patients with native hydronephrosis also showed a tendency toward multifocal lesions upon presentation (47.8%). Post-KT UBUC is characterized by pathologically aggressive and multiple foci lesions. Native kidney hydronephrosis may be a deciding factor of post-KT UBUC.