Implantable electronic medical devices are used in functional mapping of the brain before surgery and to deliver neuromodulation for the treatment of neurological and neuropsychiatric disorders. Their electrode arrays are assembled by hand, and this leads to bulky form factors with limited flexibility and low electrode counts. Thin film implants, made using microfabrication techniques, are emerging as an attractive alternative, as they offer dramatically improved conformability and enable high density recording and stimulation. A major limitation of these devices, however, is that they are invisible to fluoroscopy, the most common method used to monitor the insertion of implantable electrodes. Here, we report the development of mechanically flexible x-ray markers using bismuth- and barium-infused elastomers. We explore their x-ray attenuation properties in human cadavers and show that they are biocompatible in cell cultures. We further show that they do not distort MRI images and demonstrate their integration with thin film implants. This work removes a key barrier for the adoption of thin film implants in brain mapping and in neuromodulation. This article is protected by copyright. All rights reserved.