In this Letter we report the plasmon-enhanced upconversion in single NaYF(4) nanocrystals codoped with Yb(3+)/Er(3+). Single nanocrystals and gold nanospheres are investigated and assembled in a combined confocal and atomic force microscope setup. The nanocrystals show strong upconversion emission in the green and red under excitation with a continuous wave laser in the near-infrared at 973 nm. By the use of the atomic force microscope, we couple single nanocrystals with gold spheres (30 and 60 nm in diameter) to obtain enhanced upconversion emission. An overall enhancement factor of 3.8 is reached. A comparison of time-resolved measurements on the bare nanocrystal and the coupled nanocrystal-gold sphere systems unveil that faster excitation as well as faster emission occurs in the nanocrystals.