Single-crystal, metastable, hexagonal In2O3 (H-In2O3) nanofibers with an average diameter of 80 nm and length of up to several micrometers were synthesized on a large scale, for the first time under ambient pressure, by annealing InOOH nanofibers at 490 °C. The InOOH nanofibers were prepared by a controlled hydrolysis solvothermal reaction, using InCl3·4H2O as the starting material and ether as the solvent, in the temperature range of 190–240 °C. The solvent has significant effects on the formation of the metastable phase and the morphology of the In2O3 nanocrystals during the synthesis of the precursor InOOH. Room-temperature optical absorption spectra of the hexagonal In2O3 nanofibers showed strong absorption peak located at 325 nm (3.83 eV) with a slight blue-shift compared with that of bulk In2O3 (3.75 eV). The H-In2O3 nanofibers photoluminesce at room temperature with emission peaks at 378 nm, 398 nm, and 420 nm. The successful production of metastable hexagonal In2O3 nanofibers in large scale under mild conditions could be of interest both for applications and fundamental studies.