Applications of quantum optics in boson sampling or quantum information processing require quantum interference in high-dimensional optical networks. To this end, we study massively multichannel optical networks, containing millions of transmission channels. These massively multichannel networks can for instance be constructed by opaque scattering media. Adaptive spatial modulation of the light incident on these networks allows for complete control over the channels. Consequently, the functionality of the optical network is fully programmable. By sending multiple photons from a spontaneous parametric down-conversion source through these networks we observe quantum interference, which can be freely tuned and programmed.