This paper presents the synthesis, identification, and characterization of three novel phases in the ternary system Sm-Co-Ga: SmCoGa5 (tP7, HoCoGa5 type, tetragonal P4/mmm, Z = 1, a = 4.2419(3) angstrom, and c = 6.8559(5) angstrom), Sm4Co3Ga16 (tP23, tetragonal P4/mmm, Z = 1, a = 6.0620(5) angstrom, and c = 11.1495(9) angstrom), and SmCoGa4 (oC24, YNiAl4 type, orthorhombic Cmcm, Z = 4, a = 4.1246(6) angstrom, b = 15.608(2) angstrom, and c = 6.4556(9) angstrom). The structure of SmCoGa5 was obtained from a multiphase X-ray powder Rietveld refinement whereas the crystal structures of the other two phases were determined from single-crystal X-ray analysis. Electronic structures were calculated for all phases by first-principles DFT methods. The atomic arrangements and bonding are discussed on the basis of the partial anionic networks involving Co and Ga atoms, and a strong structural correlation is observed between SmCoGa5 and Sm4Co3Ga16. The latter, which displays paramagnetic behavior, has a resistivity of 4.2 mu Omega center dot cm at 3 K and undergoes a superconducting transition at 2.8 K.