Genetically-encoded calcium indicators (GECIs) have revolutionized neuroimaging by enabling mapping of the activity of entire neuronal populations in vivo. Visualization of these powerful activity sensors has to date been limited to depth-restricted microscopic studies due to intense light scattering in the brain. We demonstrate, for the first time, in vivo real-time volumetric optoacoustic monitoring of calcium transients in adult transgenic zebrafish expressing the GCaMP5G calcium indicator. Fast changes in optoacoustic traces associated with GCaMP5G activity were detectable in the presence of other strongly absorbing endogenous chromophores, such as hemoglobin. The new functional optoacoustic neuroimaging method can visualize neural activity at penetration depths and spatio-temporal resolution scales not covered with the existing neuroimaging techniques.