Background: An accurate and non-invasive test for diagnosing bladder cancer (BCa) is needed. Potential diagnostic markers include cancer-specific proteins in urinary extracellular vesicles (uEVs) secreted directly from BCa. In this study, we aimed to identify a uEV-based protein biomarker for the diagnosis of BCa and develop an ELISA for clinical application. In addition, we evaluated whether the identified EV protein exerts a cancer-promoting effect on BCa. Methods: Urinary EVs were isolated by ultracentrifugation with a sucrose cushion. Shotgun proteomics (TMT-labeled LC-MS/MS) was performed on uEVs from seven patients with BCa and four healthy individuals. Biomarker candidates were selected from the identified proteins and validated with targeted proteomics (SRM/MRM) of uEVs from 49 patients with BCa and 48 individuals without BCa. In addition, we developed an ELISA, without time-consuming ultracentrifugation, for quantifying the uEV-based protein biomarker and evaluated its diagnostic performance using urine samples from 46 patients with BCa and 46 individuals without BCa. The effect of the validated EV protein on BCa was also analyzed in vitro. Results: Shotgun proteomics identified 1960 proteins, of which 13 membrane proteins were significantly upregulated in the uEVs from patients with BCa. Among them, eight proteins were validated by target proteomics, and Ephrin type-A receptor 2 (EphA2) showed the best diagnostic performance (ROCAUC = 0.79). We developed EV-EphA2-CD9 ELISA as a simple assay for uEV-EphA2 measurement with clinical value, which showed good diagnostic performance (sensitivity: 60.9 %, specificity: 97.8%). Furthermore, the combination of EV-EphA2-CD9 ELISA and urine cytology improved the detection of BCa (sensitivity, 80.4 %; specificity, 97.8 %), which could reduce the frequency of cystoscopy at bladder cancer diagnosis and during follow-up. In addition, we demonstrated that EphA2 promotes the proliferation, invasion, and migration of BCa cells and that EV-EphA2 promotes the invasion of BCa cells. The invasion-promoting effect of EV-EphA2 was attenuated by knockdown of Ephrin-A1, the ligand of EphA2, on BCa cells or by prior exposure of Ephrin-A1 to EV-EphA2, suggesting that the signalling pathway via Ephrin-A1 toward BCa cells was involved in the cancer-promoting effect of EV-EphA2. Conclusions: We identified uEV-EphA2 as a novel diagnostic marker for BCa and established EV-EphA2-CD9 ELISA for uEV-EphA2 quantification, which enables the non-invasive early diagnosis of BCa in clinical practice. Furthermore, we revealed that exosomal EphA2 promotes the invasion of BCa via its ligand; Ephrin-A1 on BCa cells, suggesting that these may be potential therapeutic targets.