We report on the observation of the Seebeck ratchet effect. The effect is measured in semiconductor heterostructures with a one-dimensional lateral potential excited by terahertz radiation. The photocurrent generation is based on the combined action of a spatially periodic in-plane potential and a spatially modulated light, which gives rise to a modulation of the local temperature. In addition to the polarization-independent current due to the Seebeck ratchet effect, we observe a photon helicity dependent response and propose a microscopic mechanism to interpret the experimental findings.