A highly efficient three-dimensional nanoplasmonic coupler connecting dielectric waveguide and plasmonic waveguide constructed with a semi-elliptical structure has been proposed. Numerical simulation based on Finite Integration Technique (FIT) has been exploited for the characterization and optimization of the performance of the device. Performance optimization has been done in terms of coupling efficiency through systematic adjustments in the structural parameters of the proposed device and a coupling efficiency as high as 86% has been recorded at the telecom wavelength (1.55). Further analysis of the coupler has been presented in detail with four figures of merit (i.e. the reflection coefficient, Voltage Standing Wave Ratio (VSWR), return loss, and mismatch loss). Tolerance limit has also been added for the device to establish this work as a guiding manual for practical fabrication and nanoscale integration of the proposed coupler.