In this letter, we present a comparative experimental-simulation study of Au-nanodisc-enhanced upconversion of 1500 nm light in an Er 3+ doped TiO 2 thin film. The geometry of the Au nanodiscs was guided by finite-element simulations based on a single nanodisc in a finite computational domain and controlled experimentally using electron-beam lithography. The surface-plasmon resonances (SPRs) exhibited a well-known spectral red shift with increasing diameter, well explained by the model. However, an experimentally observed double-peak SPR, which resulted from inter-particle interactions, was expectedly not captured by the single-particle model. At resonance, the model predicted a local-field enhancement of the upconversion yield, and experimentally, the luminescence measurements showed such enhancement up to nearly 7 fold from a nanodisc with 315 nm diameter and 50 nm height. The upconversion enhancement agreed qualitatively with the theoretical predictions, however with 3-5 times higher enhancement, which was attributed to scattered light from neighboring particles.