Intravesical　instillation　therapy　is　increasingly　recognized　as　one　of　the　most　common　clinical　treatment　strategies　for　bladder　cancer.　However,　the　antitumor　efficacy　of　chemotherapy　drugs　is　still　limited　due　to　their　rapid　clearance　by　periodic　urination.　To　circumvent　this　issue,　a　drug-loaded　thin　film　comprising　the　self-assembly　of　tannic　acid　(TA)　and　ferric　ions　(Fe3+)　was　in　situ　fabricated　on　the　bladder　wall　in　vivo.　As　expected,　the　TA@Fe　film　with　adjustable　thickness　could　effectively　prolong　the　residence　time　of　anticancer　drugs　in　the　bladder　and　realize　sustained　release　of　anticancer　drugs.　Together　with　the　antibacterial　properties,　the　TA@Fe　film　enabled　improved　chemotherapeutic　efficacy.　Moreover,　the　TA@Fe　film　caused　no　adverse　effects　on　bladder　function,　demonstrating　the　in　vivo　biocompatibility.　In　addition,　the　T2　contrast　effect　of　Fe3+　was　employed　to　real-time　monitor　the　disassembly　of　the　TA@Fe　film　and　the　ensuing　drug　release　process　by　magnetic　resonance　imaging.　We　believe　that　the　TA@Fe-based　drug　delivery　platform　with　enhanced　retention　in　the　bladder　would　be　of　great　potential　for　treating　various　bladder　diseases.　©　2022　American　Chemical　Society