Kidney disease is a worldwide health hazard. Noninvasive imaging modalities such as computed tomography are often used for diagnosis, to guide treatment, and to assess a disease state over the long-term. The physiology of the kidneys can be assessed with contrast agents. We present an albumin-stabilized Au cluster with red fluorescence and robust X-ray attenuation. In vivo studies revealed distribution of the Au clusters in the liver, spleen, and kidneys, with excretion mostly via the kidneys. Under optimal conditions, this agent can outline the anatomy of mouse kidneys on 2D and 3D computed tomography imaging, with clear visualization of the renal collecting system and ureters. This is a promising agent for kidney visualization and disease diagnosis.