Pure green light emitting diodes (LEDs) are essential to realize an ultra-wide color gamut in the next-generation displays, as is defined by the Rec. 2020 standard. However, because the human eye is more sensitive to the green spectral region, it is not yet possible to achieve an ultra-pure green electroluminescence (EL) with sufficiently narrow bandwidth that covers >95% of the Rec. 2020 standard in the CIE 1931 color space. Here, we synthesize quantum-confined, two dimensional (2D) formamidinium lead bromide (FAPbBr₃) hybrid perovskites and demonstrate efficient, ultra-pure green EL for the first time. Through the dielectric-quantum-well (DQW) engineering, the quantum-confined 2D FAPbBr₃ perovskites exhibit a high exciton binding energy of 162 meV, resulting in a high photoluminescence quantum yield (PLQY) of ~92% in the spin-coated films. Our preliminary LED devices show a maximum current efficiency of 10.0 cd A^-1, comparable to the highest values reported in the perovskites LEDs. More importantly, we report the color coordinates of (0.168, 0.773), which covers 97% of the Rec. 2020 standard in the CIE 1931 color space, and represents the “greenest” LEDs ever reported. We further demonstrate large-area (3 cm²) and ultra-flexible (bending radius of 2 mm) LEDs based on the 2D perovskites.