[en] The structure of the Y-phase in the Mg-Ni-Sn system with Mg_74.5Ni_14.5Sn₁₁ composition was solved by applying the direct methods refined by a standard least square procedure. The space group is N deg. 101 (P4₂ cm) and the magnitude of the cell parameters are a=b=9.851(2) A and c=6.868(2) A. The final reliability factor is R=0.04 for 1575 independent reflections. The structure can be described in terms of an ideal structure containing two chains of cubes of Mg aligned along the c-direction. In each chain, Sn (resp. Ni) atoms occupy different opposite faces for adjacent cubes forming a stacking of tetrahedra along the c-direction. An alternative description in terms of two distorted polyhedral units is also given and the structure is compared with related inter-metallic compounds