We demonstrate a new type of laser composed of an array of coupled photonic crystal nanocavities that enables high differential quantum efficiency and output power, together with a low threshold power comparable to those of single photonic crystal cavity lasers. In our experiment, the laser efficiency increases faster than the lasing threshold with an increase in the number of coupled cavities. We observe a single mode lasing and measure the output powers that are two orders of magnitude higher than in single nanocavity lasers. Finally, we study the laser behavior theoretically and show that the benefits resulting from the coupling of cavities are due to strong cavity effects such as the enhanced spontaneous emission rate. (c) 2005 Optical Society of America.