Abstract: The crystal of potassium terbium metaphosphate KTb(PO_3)_4 belongs to the monoclinic space group P2_1 with unit cell parameters a=0.7239(1) nm, b=0.8325(2) nm, c=0.7896(1) nm, β=91.82(1)°, V=0.4756 nm, Z=2. The structure was solved by heavy-atom method and the final R=0.034 after refining by block-diagonal least-squares method. The structure contains helical ribbons, (PO_3)_n, formed by cornersharing tetrahedra of PO_4. The coordination of the terbium by oxygen atoms is eight-fold with the geometry of dodecahedra. The shortest Tb—Tb distance is 0.6544 nm.