The optical spectrum of diatomic IrSi has been investigated for the first time, with transitions observed in the range from 17,178 to 23,858 cm(-1) (582-419 nm). A rich spectrum has been recorded, consisting of 14 electronic band systems and a number of unclassified bands. Thirty-one bands have been investigated with rotational resolution, allowing the ground state to be identified as X(2)Δ5∕2 arising from the 1σ(2)1π(4)2σ(2)1δ(3)3σ(2) configuration. The ground X(2)Δ5∕2 state is characterized by ΔG1∕2 = 533 cm(-1) and r0 = 2.0899(1) Å for the more abundant isotopic form, (193)Ir(28)Si (57.8%). The measured excited electronic states have equilibrium bond lengths ranging from 2.17 to 2.25 Å and vibrational frequencies ranging from 365 to 452 cm(-1). Ab initio calculations were also carried out on the molecule using the complete active space self-consistent field and multistate complete active space second-order perturbation theory methods, with relativistic and spin-orbit effects included through the restricted active space state-interaction with spin-orbit coupling method. The calculated ground state agrees with experiment, and a large number of excited states lying within 20,000 cm(-1) of the ground state are reported.