Self-terminated electrodeposition process is used to grow ultrathin catalytic iridium (Ir) films. With a simple potential pulsing method, semi-coherent Ir films are deposited on various substrates in K₃IrCl₆-Na₂SO₄ electrolytes at the temperature range between 40 °C and 70 °C. The electrochemical reduction of IrCl_6-x·H₂O_x^3-x to Ir is quenched at the onset potential of hydrogen evolution reaction, even though the deposition overpotentials exceed 1.0 V because the deposited Ir surfaces are immediately passivated by H adsorption which can prevent the further Ir deposition. However, the deposited Ir film surfaces are reactivated toward further electrochemical reduction of IrCl_6-x·H₂O_x^3-x to Ir when the potential is rapidly changed to positive value where the adsorbed H is oxidized. After 1 deposition pulse, the X-ray photoelectron spectroscopy (XPS) derived Ir thickness is 0.085 ± 0.028 nm based on uniform overlayer model which is mostly independent deposition time. With repeated potential pulses, the XPS derived Ir thickness is increased from 0.343 ± 0.012 nm (2 deposition pulses) to 3.143 ± 0.083 nm (10 deposition pulses). The Ir coverage determined by H under potential deposition (H_UPD) approaches a monolayer coverage after 3 deposition pulses. The various combination of Ir-based ultrathin films prepared self-terminated electrodeposition demonstrate high activity for hydrogen catalysis in alkaline and oxygen catalysis in acid.