Proton radiography is a potentially valuable tool for the image guidance of proton therapy cancer treatment. While proton therapy is desirable due to its high dose deposition accuracy, proton radiography, which could, in theory, be applied simultaneously to treatment, has an intrinsically low soft tissue contrast, making it difficult to visualize tumors. To enhance this tumor contrast, high-Z nanoparticles could be targeted to a tumor before imaging. To assess the efficacy of gold as a contrast agent, phantoms consisting of gold leaf mounted on acrylic backing were developed to simulate a tumor tagged with gold nanoparticles (AuNPs). Calculations are presented to correlate a given thickness of gold with scenarios for tagging cancers with AuNPs, in terms of the size of the functionalized nanoparticles, the diameter of the tumor, as well as the efficiency by which the nanoparticles are taken up by the malignant cells. These calculations determined phantoms that best describe particular tagging conditions, and are also applicable to ex vivo specimens made by injecting AuNPs into a mouse model. Using a ×3 proton magnifying lens with the 800-MeV LANSCE proton beam, a 1-μm-thick Au foil was radiographically discernible within 1 cm of acrylic, representing sensitivity to a material percent density change of 0.2%. This indicates that AuNP-enhanced proton radiography could be used to characterize small tumors, allowing for early detection and treatment of malignant tissues, as well as for in situ imaging for enhanced treatment localization.