Implantable neuroprosthetic devices such as electrocorticography (ECoG) and intracortical microelectrodes have the potential to restore neurological functions in disabled individuals. They constitute the direct interface with the body and require long term efficiency. We propose an ultra-thin diamond-based implant to tackle challenges related with long term implantation, and demonstrate its functionality on rodents. Here we take advantage of thin-film diamond to generate not only the electrodes but also the thin passivation foil, leading to a full thin film diamond EcoG. Implants were fabricated based on boron-doped diamond (BDD) electrodes connected through titanium nitride (TiN) conductive tracks, all being encapsulated in intrinsic diamond. A complete electrochemical characterization proved that the TiN tracks were well embedded inside diamond and the efficiency of the BDD electrodes. Functionality of the ECoG device was also provided by the recording of classic visual evoked potentials (VEPs) on wild-type mice and rats. This thin film diamond technology successfully responds to long term issues (linked with electrode material or device packaging) and holds the potential to inspire and pave the way for future generations of various electrode arrays.