A multivariate sensor (MVS) is described for measurement of melt temperature, melt pressure, melt velocity, and melt viscosity. Melt pressure and temperature are respectively obtained through the incorporation of a piezo-ceramic element and infrared thermopile. Melt velocity is derived from the initial response of the melt temperature as the polymer melt flows across the sensor lens. The apparent melt viscosity is then derived based on the melt velocity and the time derivative of the increasing melt pressure. The response of the MVS is analyzed using an instrumented mold including piezoelectric pressure sensors, an infrared pyrometer, and thermocouples. A 12-run, blocked half-fractional design of experiments (DOE) was run to characterize the effect of melt temperature, mold temperature, packing pressure, and ram velocity. The results show that the MVS provides excellent measurement of melt temperature and pressure. The accuracy of the melt velocity estimations depended on the ram velocity set-point, yielding a coefficient of determination of 0.91 for the lower ram velocities, and reaching saturation for melt velocities about 450 mm/s. The apparent melt viscosity estimated by the MVS are close to those predicted by the Cross-WLF model, exhibiting appropriate shear thinning but behavior but inconsistent temperature dependence.