Bladder carcinoma (BC) is the tenth most frequent malignancy worldwide, with high morbidity and mortality rates. Despite recent treatment advances, high-grade BC and muscle-invasive BC present with significant progression and recurrence rates, urging the need for alternative treatments. The microRNA-21 (miR-21) has superexpression in many malignancies and is associated with cellular invasion and progression. One of its mechanisms of action is the regulation of RECK, a tumor suppressor gene responsible for inhibiting metalloproteinases, including MMP9. In a high-grade urothelial cancer cell line, we aimed to assess if miR-21 downregulation would promote RECK expression and decrease MMP9 expression. We also evaluated cellular migration and proliferation potential by inhibition of this pathway. In a T24 cell line, we inhibited miR-21 expression by transfection of a specific microRNA inhibitor (anti-miR-21). There were also control and scramble groups, the last with a negative microRNA transfected. After the procedure, we performed a genetic expression analysis of miR-21, RECK, and MMP9 through qPCR. Migration, proliferation, and protein expression were evaluated via wound healing assay, colony formation assay, flow cytometry, and immunofluorescence.After anti-miR-21 transfection, miR-21 expression decreased with RECK upregulation and MMP9 downregulation. The immunofluorescence assay showed a significant increase in RECK protein expression (p <
0.0001) and a decrease in MMP9 protein expression (p = 0.0101). The anti-miR-21 transfection significantly reduced cellular migration in the wound healing assay (p <
0.0001). Furthermore, in the colony formation assay, the anti-miR-21 group demonstrated reduced cellular proliferation (p = 0.0008), also revealed in the cell cycle analysis by flow cytometry (p = 0.0038). Our results corroborate the hypothesis that miR-21 is associated with BC cellular migration and proliferation, revealing its potential as a new effective treatment for this pathology.