Electronic structure and spectroscopic properties of the ground and low-lying excited states of Sit Si within 4 eV have been investigated by using a multireference singles and doubles configuration interaction (MRDCI) method that includes relativistic effective core potentials. Potential energy curves of a number of A-S states of singlet, triplet, and quintet spin multiplicities are constructed. Spectroscopic parameters (T-e, r(e), omega(e), D-e, and mu(e)) of 27 bound Lambda-S states are reported. The ground state of SnSi belongs to the X-3 Sigma(-) symmetry with an estimated dissociation energy (D-e) of 2.49 eV. However, with the inclusion of the spin-orbit coupling, D-e reduces to 2.11 eV. Spectroscopic properties of at least 36 Omega states are determined. Transition probabilities of several singlet-singlet and triplet-triplet transitions are calculated. Partial radiative lifetimes of some of these transitions are estimated. A number of weak Omega-Omega transitions with partial radiative lifetimes of the order of milliseconds or more is also predicted here.