In this study, we successfully optimized the ‘pre-sintering-hot-isostatic pressing’ (HIPing) combined process, yielding infrared (IR)-transparent Y₂O₃ ceramics with a homogeneous submicron grain size. Green bodies were pre-sintered in a two-step process in a vacuum furnace under various sintering conditions to obtain a high enough density with restrained grain growth for a subsequent HIPing treatment. The effects of the pre-sintering conditions on the density and grain size of the samples were analyzed in detail. It was found that during the pre-sintering process, dramatic grain growth occurs from the surfaces of the samples when the density reaches about 95%, after which it spreads over the entire sample. For a sample pre-sintered at a T₁ temperature of 1700 °C and a T₂ temperature of 1550 °C for 5 h, after annealing in air and subsequent HIPing, good IR-transparency (i.e., transmittance of a 1-mm-thick sample at 2500 nm of 73%) was achieved without significant grain growth (average grain size of 0.37 μm).