A high power optically-pumped vertical-external-cavity surface-emitting laser with a diamond heatspreader is demonstrated. Owing to the good thermal conductivity, diamond can accelerate the dissipation of heat in the active region and increase the output power significantly. The effects of the curvature radius, the transmission of output coupler, the spot size of the pump, and the temperature of the heat sink on the output power are investigated. A maximum output power of 880 mW is obtained under the conditions of 10℃ temperature, 15-mm curvature radius, 3% transmission of the output coupler, and 8500-mW pump power. The slope efficiency and the optical-to-optical conversion efficiency of the laser are about 17% and 15%, respectively.