We report measurements of DC (AC) magnetic susceptibility and neutron diffraction on TbNiAl2 alloy. The Rietveld refinements of the x-ray and neutron diffraction data are consistent with an orthorhombic structure of the type MgCuAl2 (space group Cmcm). The results of DC (AC)- magnetic susceptibility show two successive magnetic transitions at 20 K and 11.7 K with antiferromagnetic and ferromagnetic (or ferrimagnetic) features, respectively. On the other hand, neutron diffraction patterns show that, below 20 K and down to 12 K, new reflexions appear, confirming the antiferromagnetic character of the transition observed in the macroscopic measurements. Also, at least one of these new reflexions, located at Q 1.2 Å-1, shifts to higher angles when the temperature decreases, indicating an incommensurate magnetic structure. Below 11 K, many reflexions disappear and new reflexions increase, evidencing a new magnetic transition. © Published under licence by IOP Publishing Ltd. ; This work was supported by MAT 2008-06542-C04 and MAT2011-27573-C04 projects. The authors thank ILL and CRG-D1B for allocating neutron beam time