Halide double perovskites have recently emerged as a promising environmentally friendly optoelectronic and photovoltaic material for their inherent thermodynamic stability, high defect tolerance, and appropriate band gaps. However, to date, no ferroelectric material based on halide double perovskites has been discovered. Herein, by hetero-substitution of lead and cation intercalation of n-propylamine, the first halide double perovskite ferroelectric, (n-propylammonium)(2)CsAgBiBr₇ (1), is reported and it exhibits distinct ferroelectricity with a notable saturation polarization of about 1.5 mu C cm(-2). More importantly, single-crystal photodetectors of 1 exhibit extraordinary performance with containing high on/off ratios of about 10(4), fast response rates of 141 mu s, and detectivity as high as 5.3x10(11) Jones. This finding opens a new way to design high-performance perovskite ferroelectrics, and provides a viable approach in the search for stable and lead-free optoelectronic materials as an alternative to the lead-containing system.