In this paper, we present the conception, fabrication and characterization of a thermopile designed to detect terahertz electromagnetic fields at room temperature. The thermopile is made of four doped silicon/titanium thermocouples. The absorber consists of a metallic grid made of titanium, deposited at the same time as the metal part of the thermocouples. The design of the grid is based on a theoretical multilayer model using equivalent resistivity and taking into account small diffraction effects. The grid is deposited on a 2.4 mm x 2.4 mm silicon nitride square membrane. The time constant of the sensor is measured at 0.3 THz to be 10 ms, which is consistent with finite elements simulations. The responsivity is evaluated at 4.8 mu V/(W m(-2)). Due to a large impedance, which leads to a large Johnson noise, the noise equivalent power is 1.5 x 10(-6) W Hz(-1/2).