Optical neurotechnologies use light to interface with neurons and can monitor and manipulate neural activity with high spatial-temporal precision over large cortical areas. There has been considerable progress in miniaturizing microscopes for head-mounted configurations, but existing devices are bulky and their application in humans will require a more non-invasive, fully implantable form factor. Here we report an ultrathin, miniaturized subdural complementary metal–oxide–semiconductor (CMOS) optical device for bidirectional optical stimulation and recording. We use a custom CMOS application-specific integrated circuit that is capable of both fluorescence imaging and optogenetic stimulation, creating a probe with a total thickness of less than 200 µm, which is thin enough to lie entirely within the subdural space of the primate brain. We show that the device can be used for imaging and optical stimulation in a mouse model and can be used to decode reach movement speed in a non-human primate.