The isothermal section of the La–Ni–Zn system at 400 °C was studied from 16.7 to 100 at.% La, by coupling X-ray powder diffraction and SEM–EDXS analyses on induction melted samples. Several binary phases were found to dissolve an appreciable amount of the third element forming ternary solid solutions which extend along lines at a constant La content. Particularly, a continuous solid solution was detected between the LaNi5 and LaZn5 isostructural compounds. Seven ternary phases were found to exist at 400 °C, both stoichiometric and showing homogeneity ranges at a constant La concentration. The crystal structures of LaNiZn (hP9-ZrNiAl), La2Ni2Zn (oI10-Pr2Ni2Al) and La3Ni3Zn (oS28-Y3Co3Ga) were confirmed; quite extended homogeneity ranges were detected for the last two phases. The crystal structure of the new phase of stoichiometry La7Ni2Zn was solved ab initio from X-ray powder diffraction data: it crystallizes in its own structure type, tetragonal, P4/mbm, tP20, a = 1.24104(9) nm, c = 0.38256(3) nm. A phase of general formula La(Ni,Zn)3 extends from about 2.5 to 9 at.% Zn whose crystal structure corresponds to the hexagonal CeNi3 type: a negligible addition of Zn is likely to stabilize this structure instead of the analogue rhombohedral PuNi3 type, which the binary compound LaNi3 crystallizes in. Two more ternary phases were found in the isothermal section at 400 °C: the stoichiometric ∼La3Ni4Zn3 and the LaNi1+xZn2−x (0 < x<∼0.5) phase extending along the 25 at.% La isoconcentrate line.