We describe a method to enhance the light output power of nitride-based light-emitting diodes (LEDs) through the coupling of multiple quantum wells (MQWs) with localized surface plasmon (LSP). The LSP was generated on an Ag nanotriangle array (NTA) on a 40-nm-thick p-type GaN layer beneath the p-pad of the LED, which was partially etched by inductively coupled plasma system. The Ag NTA was fabricated by nanosphere lithography. The resonant frequency of a generated LSP can be precisely controlled by changing the size of the polystyrene nanosphere and the Ag deposition thickness. Under the optimum conditions, the light output power of LED with an Ag NTA was 15.4% higher than LED without an Ag NTA at an inject current of 20 mA. The improvement in light output power can be attributed to the coupling effect between MQW and LSP.