Additive manufacturing (AM), covering processes frequently referred to as rapid prototyping, rapid manufacturing, or three-dimensional (3D) printing, provides new opportunities in the manufacture of highly complex and custom-tting medical devices and products. Although many medical applications of AM have been developed and physical properties of the resulting parts have been studied, the characterization of AM materials using computed tomography (CT) has not been explored. The aim of this study was to determine the CT number of commonly used AM materials. There are many potential applications of the information resulting from this study in the design and manufacture of wearable medical devices, implants, prostheses, and medical imaging test phantoms. Twenty-nine AM material samples were CT scanned and the resultant images analyzed to ascertain the materials’ CT numbers and appearance in the images. It was found that some AM materials have CT numbers very similar to human tissues; that fused deposition modeling, stereolithography, and selective laser sintering produce samples that appear uniform on CT images; and that 3D printed materials show a variation in internal structure. AM materials may be suitable for the development of anatomically accurate phantoms for image quality and radiation dose tests.