X‐ray detectors with high sensitivity are of great significance in both civil and military fields. Over the past decades, great efforts have been made to improve the sensitivity in conventional inorganic materials, but mainly at the cost of increasing the energy consumption with a quite high operating voltage. Developing photosensitive ferroelectrics directly as detector materials may be a conceptually new strategy in view of the strong ferroelectric spontaneous polarization (P_s) that assists photoinduced carriers separation and transport. A high‐performance X‐ray detector in 2D hybrid halide perovskite ferroelectric (C₄H₉NH₃)₂(C₂H₅NH₃)₂Pb₃Br₁₀ (BA₂EA₂Pb₃Br₁₀) (P_s = 5 µC cm⁻²) is fabricated and exhibits an ultrahigh X‐ray sensitivity up to 6.8 × 10³ µC Gy_air⁻¹ cm⁻² even at a relatively low operating voltage, which is over 300‐fold larger than that of state‐of‐the‐art α‐Se X‐ray detectors. Such a brilliant figure‐of‐merit is largely attributed to the superior mobility–lifetime products associated with the strong ferroelectric polarization of BA₂EA₂Pb₃Br₁₀. As pioneering work, these findings inform the exploration of hybrid halide perovskite ferroelectrics toward high‐performance photoelectronic devices.