Thehydride NdMnSiH obtained by exposure of the ternary silicide NdMnSiunder a pressure of 4 MPa of hydrogen at 523K crystallizes in the tetragonal ZrCuSiAs-type structure where H atomoccupies the tetrahedral [Nd₄] sites. The hydrogenation of NdMnSi inducesan increase in the unit cell volume close to 3.3%.The investigation of NdMnSiH by magnetization and specific heat measurementsreveals the existence of two antiferromagnetic ordering, respectively, at <i>T</i>_<i>N</i>1=565(5)  Kand <i>T</i>_<i>N</i>2=103(4)  K. Neutron powder diffraction shows that these Néel temperaturesare associated with an antiferromagnetic arrangement of the (i) Mnsubstructure (<i>T</i>_<i>N</i>1) and (ii) Nd substructure linked to a reorientationof the Mn one (<i>T</i>_<i>N</i>2). Comparison of the <i>T</i>_<i>N</i>1 and<i>T</i>_<i>N</i>2 temperatures of NdMnSiH to those reported for the initialcompound NdMnSi indicates a strong increase in <i>T</i>_<i>N</i>1 (280  K -> 565  K)and a significant decrease in <i>T</i>_<i>N</i>2 (185  K -> 03 K). The magnetic properties, magnetic structures, and values of Nd and Mn ordered magnetic moments are discussed using both band structure calculations and comparison with the behaviors of other ternary silicides and germanides based on manganese and rare earth elements.< Réduire