Citrus fruits such as lemon, orange, grapefruit, and tangerine are consumed for their flavor, low cost, and human health benefits. However, citrus juice extraction generates by-products that are mostly unused and is discharged in landfills. In this study, the by-products of lemon, orange, grapefruit, and tangerine were subjected to solid-state fermentation (SSF) using Fusarium oxysporum, Penicillium purpurogenum GH2, Trichoderma harzianum T1-04, and Aspergillus niger GH1 to enhance their antioxidant activity. After fermentation, ethanol extracts were obtained and tested for their antioxidative activity by employing three techniques, 2,2-diphenyl-1-picrylhydrazyl (DPPH˙), ferric reducing antioxidant power (FRAP), and lipid oxidation inhibition (LOI). An increase in antioxidant activity from 33.13 to 41.62 mg/g of antioxidants after fermentation of tangerine by-products by A. niger GH1 was observed. Major compounds present in ethanol extracts obtained after fermentation by A. niger GH1were identified by HPLC-MS, and their m/z corresponded to chlorogenic acid, didymin, naringin, and hesperidin. These results indicated that SSF is a suitable method to enhance antioxidant activity of citrus by-products.