Background: Competence in transurethral resection of bladder tumors (TURB) is critical in bladder cancer management and should be ensured before independent practice. Objective: To develop an assessment tool for TURB and explore validity evidence in a clinical context. Design, Setting, and Participants: From July 2019 to March 2021, a total of 33 volunteer doctors from three hospitals were included after exemption from the regional ethics committee (REG-008-2018). Participants performed two TURB procedures on patients with bladder tumors. A newly developed assessment tool (Objective Structured Assessment for Transurethral Resection of Bladder Tumors Skills, OSATURBS) was used for direct observation assessment (DOA), self-Assessment (SA), and blinded video assessment (VA). Outcome Measurements and Statistical Analysis: Cronbach's alpha and Pearson's r were calculated for across items internal consistency reliability, inter-rater reliability, and test-retest reliability. Correlation between OSATURBS scores and the operative experience was calculated with Pearson's r and a pass/fail score was established. Differences in assessment scores were explored with paired t-Test and independent samples t-Test. Results and Limitations: The internal consistency reliability across items Cronbach's alpha was 0.94 (n = 260, p < 0.001). Inter-rater reliability was 0.80 (n = 64, p < 0.001). Test-retest correlation was high, r = 0.71 (n = 32, p < 0.001). Relationship with TURB experience was high, r = 0.71 (n = 32, p < 0.001). Pass/fail score was 19 points. DOAs were strongly correlated with video ratings (r = 0.85, p < 0.001) but with a significant social bias with lower scores for inexperienced and higher scores for experienced participants. Participants tended to overestimate their own performances. Conclusions: OSATURBS tool for TURB can be used for assessment of surgical proficiency in the clinical setting. DOA and SA are biased, and blinded VA of TURB performances is advised.