The magnetic properties of TbSi (Pnma space group, FeB-type of structure, T N = 57 K) were reinvestigated by means of powder neutron diffraction in the temperature range 1.5–70 K. Our results show that the previously reported collinear antiferromagnetic structure with wavevector q = (0, 1 2 ,0) is stable only in the low-temperature (l.t) range. At T c = 36 K there is a first-order transition associated with a drastic symmetry reduction to a three-dimensionally ( q = ( q x , q x , q z )) modulated magnetic phase which remains incommensurate with the crystal lattice in the whole high-temperature (h.t) region up to T N = 60 K. It follows from the present data analysis that the dominant wavevector component is q z , but the incommensurateness also involves the q x and q y components. The wavevector length varies strongly in the range 36–40 K and less strongly in the range 40–42 K. Indications are given that the motion of the wavevector in reciprocal space in the region 36–42 K has a rotative component within the plane (001). Above 42 K it tends to the commensurate value q≈(0, 1 2 , 1 8 ).