Objective: This study evaluated the performance of the novel liquid fiducial marker (BioXmark®) in IGRT for bladder cancer. Methods: 20 patients with muscle invasive bladder cancer were entered in this prospective, single center, Phase I-II study. The novel BioXmark^® liquid markers were injected around the tumor using a flexible cystoscopy. Visibility and stability of the markers were evaluated on planning-CT and CBCT. Prospectively defined threshold for success was set at a visibility of 75%. Results: In total, 76 markers were implanted in 20 patients. Of those, 60 (79% 95% CI ± 9%) were visible on CT scan. Due to the learning curve of the technique, the visibility improved in the last 75% of patients (86% visibility) compared to the first 25% of patients with 58% visibility. Concerning stability of the BioXmark^® marker, all visible markers after CT acquisition were still detectable at the last CBCT without displacement. In 15/20 (75%) of the patients, three or more markers were visible on CT. No BioXmark^® related adverse events were reported. Conclusion: The success rate of this novel fiducial marker was 79%, which is above the prospectively defined threshold rate. A distinct learning curve of the injection of the liquid marker was seen over the study period. The marker showed sustained visibility and positional stability during treatment phases and also appears to be safe and easy to inject. Advances in knowledge: This novel liquid BioXmark® marker seems to be a very promising tool in daily-adaptive IGRT for bladder preserving chemoradiotherapy in muscle invasive bladder cancer.