High-density poly(ethylene glycol) (PEG)-coated iron-oxide-gold core-shell nanoparticles (AuIONs) were developed as T-2-weighted magnetic resonance imaging (MRI) contrast agents for cancer imaging. The PEG-coated iron-oxide-gold core-shell nanoparticles (PEG-AuIONs) were approximately 25 nm in diameter with a narrow distribution. Biodistribution experiments in mice bearing a subcutaneous colon cancer model prepared with C26 murine colon adenocarcinoma cells showed high accumulation of the PEG-AuIONs within the tumor mass and low nonspecific accumulation in the liver and spleen, resulting in high specificity to solid tumors. T-2-weighted MR images following intravenous injection of PEG-AuIONs showed selective negative enhancement of tumor tissue in an orthotopic pancreatic cancer model prepared with Mia-PaCa-2 human pancreatic adenocarcinoma cells. These results indicate that PEG-AuIONs are a promising MRI contrast agent for diagnosis of malignant tumors, including pancreatic cancer.