Ferroelectricity and X‐ray detection property have been recently implemented for the first time in hybrid bromide double perovskites. It sheds a light on achieving photosensitive and ferroelectric multifunctional materials based on 2D lead‐free hybrid halide double perovskites. However, the low T_c, small P_s, and relatively low X‐ray sensitivity in the reported bromide double perovskites hinder practical applications. Herein, the authors demonstrate a novel 2D lead‐free iodide double perovskite (4,4‐difluoropiperidinium)₄AgBiI₈ (1) for high‐performance X‐ray sensitive ferroelectric devices. Centimeter‐sized single crystal of 1 is obtained and exhibits an excellent ferroelectricity including a high T_c up to 422 K and a large P_s of 10.5 μC cm⁻². Moreover, due to a large X‐ray attenuation and efficient charge carrier mobility (μ)–charge carrier lifetime (τ) product, the crystal 1 also exhibits promising X‐ray response with a high sensitivity up to 188 μC·Gy_air⁻¹ cm⁻² and a detection limit below 3.13 μGy_air·s⁻¹. Therefore, this finding is a step further toward practical applications of lead‐free halide perovskite in high‐performance photoelectronic devices. It will afford a promising platform for exploring novel photosensitive ferroelectric multifunctional materials based on lead‐free double perovskites.