Dimensionality reduction induced metal-insulator transitions in oxide heterostructures are usually coupled with structural and magnetic phase transitions, which complicate the interpretation of the underlying physics. Therefore, achieving isostructural MIT is of great importance for fundamental physics and even more for applications. Here, we report an isostructural metal-insulator transition driven by dimensional-crossover in spin-orbital coupled SrIrO₃ films. By using in situ pulsed laser deposition and angle-resolved photoemission spectroscopy, we synthesized and investigated the electronic structure of SrIrO₃ ultrathin films with atomic-layer precision. Through inserting orthorhombic CaTiO₃ buffer layers, combined low-energy electron diffraction measurements and first-principles density functional theory calculations show that the crystal structure of SrIrO₃ films remains bulklike with similar oxygen octahedra rotation and tilting when approaching the ultrathin limit. We observe that a dimensional-crossover metal-insulator transition occurs in isostructural SrIrO₃ films. Intriguingly, we find the bandwidth of J_eff = 3/2 states reduces with lowering the dimensionality and drives the metal-insulator transition. Our results establish a bandwidth controlled metal-insulator transition in the isostructural SrIrO₃ thin films.