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Simple Summary: Recently, with the consolidation of the new high-throughput sequencing techniques, the old dogma that the bladder was sterile under normal conditions has been abandoned. Thus, the urinary microbiota in patients with bladder cancer could be involved in its pathophysiology, and its modulation with probiotics could be an interesting adjuvant therapeutic target. This narrative review aims to compile all the evidence to date provided by preclinical studies, animal assays, and clinical trials on the efficacy of probiotics that are either oral or injected into the bladder, thereby reducing the tumor size, preventing recurrences, modulating the immune system, etc. Bladder cancer is around the 10th most diagnosed cancer, although has a considerable mortality. Recent research and new methodologies have discarded the historical dogma that the bladder (and urine) was sterile under normal conditions. Specifically, only a few studies have reported a detailed analysis of the urinary microbiota in patients with bladder cancer, thus exhibiting a remarkable variability due to the low biomass of the urinary microbiota and the influence of many factors. Nevertheless, this research shows us signals that urinary microbiota is a factor to be considered in the pathophysiology of bladder cancer. More importantly, probiotics could be useful as an adjuvant therapy to reduce the recurrence rate or increase the disease-free period after surgery. In vitro studies and animal assays have shown promising results, but the research in this context has also been scarce, and only a few studies have been conducted in humans. In summary, there is little evidence of the possible beneficial effect of probiotics in controlling the overgrowth of genera that could be involved in the carcinogenesis of bladder cancer. This narrative review aims to compile all the evidence to date on the therapeutic potential of probiotics injected directly into the bladder or orally administered. [ABSTRACT FROM AUTHOR]