TlBiSSe is a rare realization of a three-dimensional semimetal with a conically dispersing band that has an optical response which is well isolated from other contributions in a broad range of photon energies. We report optical and magneto-optical spectroscopy on this material. When the compound is chemically tuned into a state of the lowest carrier concentration, we find a nearly linear frequency dependence of the optical conductivity below 0.5 eV. Landau level spectroscopy allows us to describe the system with a massive Dirac model, giving a gap 2Δ=32 meV and an in-plane velocity parameter vxy=4.0×105 m/s.