Single-component emitters with stable and bright warm white-light emission are highly desirable for high-efficacy warm white light-emitting diodes (warm-WLEDs), however, materials with such luminescence properties are extremely rare. Low-dimensional lead (Pb) halide perovskites can achieve warm white photoluminescence (PL), yet they suffer from low stability and PL quantum yield (PLQY). While Pb-free air-stable perovskites such as Cs2AgInCl6 emit desirable warm white light, sophisticated doping strategies are typically required to increase their PL intensity. Moreover, the use of rare metal-bearing compounds along with the typically required vacuum-based thin-film processing may greatly increase their production cost. Herein, organic-inorganic hybrid cuprous (Cu+)-based metal halide MA(2)CuCl(3) (MA = CH3NH3+) that meets the requirements of i) nontoxicity, ii) high PLQY, and iii) dopant-free is presented. Both single crystals and thin films of MA(2)CuCl(3) can be facilely prepared by a low-cost solution method, which demonstrate bright warm white-light emission with intrinsically high PLQYs of 90-97%. Prototype electroluminescence devices and down-conversion LEDs are fabricated with MA(2)CuCl(3) thin films and single crystals, respectively, which show bright luminescence with decent efficiencies and operational stability. These findings suggest that MA(2)CuCl(3) has a great potential for the single-component indoor lighting and display applications.