The ternary rare-earth gallium antimonides, REGaSb2 (RE = La-Nd, Sm), have been synthesized through reaction of the elements. The structures of SmGaSb2 (orthorhombic, space group D-2(5)-C222(1), Z = 4, a = 4.3087(5) Angstrom, b = 22.093(4) Angstrom, c = 4.3319(4) Angstrom) and NdGaSb2 (tetragonal, space group D-4h(19)-I4(1)/amd, Z = 8,a = 4.3486(3) Angstrom, c = 44.579(8) Angstrom) have been determined by single-crystal X-ray diffraction. The SmGaSb2-type structure is adopted for RE = La and Sm, whereas the NdGaSb2-type structure is adopted for RE = Ce-Nd. The layered SmGaSb2 and NdGaSb2 structures are stacking variants of each other. In both structures, two-dimensional layers of composition (2)(infinity)[GaSb] are separated from square nets of Sb atoms (2)(infinity)[Sb] by RE atoms. Alternatively, the structures may be considered as resulting from the insertion of zigzag Ga chains between (2)(infinity)[RE Sb-2] slabs. In SmGaSb2, all of the Ga chains are parallel and the (2)(infinity)[SmSb2] layers are stacked in a ZrSi2-type arrangement. Tn NdGaSb2, the Ga chains alternate in direction, resulting in a doubling of the long axis relative to SmGaSb2, and the (2)(infinity)[NdSb2] layers are stacked in a Zr3Al4Si5-type arrangement. Extended Huckel band structure calculations are used to explain the bonding in the [GaSb2](3-) substructure.