In situ groundwater remediation often calls for a chemical or biological amendment to be injected as an aqueous solution into a contaminated groundwater aquifer. Accordingly, remediation depends on mixing the amendment into the contaminated groundwater, which, in turn, depends on spreading the plume of the injected amendment effectively. Here, we present proof-of-principle results from a laboratory study showing that amendment plume spreading can be enhanced by heating the injected water, which is consistent with the mechanism of miscible viscous fingering. The heated water has a lower viscosity, rendering a mobility ratio (i.e., log viscosity ratio) of 1.2 that generates elongated plume perimeters for essentially consistent plume areas. Using a quasi-two-dimensional apparatus and recording photographs after each increment of the injection volume, two image analysis techniques were employed to measure the area and perimeter of the injected plume, and the results are compared to isothermal controls, showing that the plume perimeter increased by 47% when determined by binary image analysis or 56% when determined by morphological image analysis. Accordingly, this study offers evidence that heating the injected water enhances miscible plume spreading in porous media.