Unlike Gd6Co5/3Si3, which is obtained as a single phase after melting of the constituents, annealing under vacuum at 1073 K for 1 month is necessary to prepare the new ternary silicide Gd6Ni5/3Si3. Indeed, after melting, this alloy is a mixture containing mainly Gd3NiSi2 and Gd5Si3. These two compounds, Gd6M5/3Si3 (M = Co or Ni), crystallize with the hexagonal Ce6Ni2−xSi3-type structure. The crystal structure of Gd6Ni5/3Si3, refined on a single crystal with space group P63/m and unit cell parameters a = 11.7433(1) Å and c = 4.1857(1) Å, is characterized by infinite chains of face-shared trigonal prisms [Gd6] filled by silicon or nickel atoms. These chains run along the c-axis and extend as triangular columns by sharing rectangular faces in the ab-plane. Between these columns, infinite chains of face-shared octahedra of Gd atoms partially are filled by Ni atoms. The strong delocalization of the electron density of Ni observed in these latter chains has been attributed to high steric strains. No diffuse scattering nor superstructure were observed from electron diffraction experiments. The magnetization measurements reveal that (i) Gd6Ni5/3Si3 orders ferromagnetically at 310 K, a Curie temperature higher than that observed for Gd6Co5/3Si3 (TC = 294 K) and pure gadolinium (TC = 294 K); (ii) Ni as well as Co carries no magnetic moment; and (iii) these ternary silicides exhibit interesting magnetocaloric properties; for instance, the magnetic entropy (ΔSm determined by magnetization measurements) of Gd6Ni5/3Si3 is at a maximum around 312 K with values of ΔSm = −2.93 and –5.72 J/K kg at applied magnetic fields of 2 and 4.8 T, respectively. These properties are compared to those existing for the most famous magnetocaloric materials as Gd or Gd5Ge2Si2.< Réduire