PURPOSE. Micro(mi)RNAs negatively regulate a wide variety of genes through degradation or posttranslational inhibition of their target genes. The purpose of this study was to investigate the role of miR-23a in modulating RPE cell survival and gene expression in response to oxidative damage. METHODS. The expression level of miR-23a was measured in macular retinal pigment epithelial (RPE) cells of donor eyes with aged-related macular degeneration (AMD) and age-matched normal eyes by using qRT-PCR. Cultured human ARPE-19 cells were transfected with miR-23a mimic or inhibitor. Cell viability was assessed by the MTT assay. Apoptosis was determined by incubating cells with hydrogen peroxide (H2O2) or t-butylhydroperoxide (tBH). Caspase-3 activity and DNA fragmentation were measured by enzyme-linked immunosorbent assays. The protein relevant to apoptosis, such as Fas expression level, was analyzed by Western blot analysis. RESULTS. miR-23a expression was significantly downregulated in macular RPE cells from AMD eyes. H2O2-induced ARPE-19 cell death and apoptosis were increased by an miR-23a inhibitor and decreased by an miR-23a mimic. Computational analysis found a putative target site of miR-23a in the 3'UTR of Fas mRNA, which was verified by a luciferase reporter assay. Forced overexpression of miR-23a decreased H2O2 or tBH-induced Fas upregulation, and this effect was blocked by downregulation of miR-23a. CONCLUSIONS. The protection of RPE cells against oxidative damage is afforded by miR-23a through regulation of Fas, which may be a novel therapeutic target in retinal degenerative diseases. (Invest Ophthalmol Vis Sci. 2011;52:6308-6314) DOI:10.1167/iovs.10-6632