Solid-state fermentation is a microbial process carried out mostly on the surface of solid materials. Grapefruit by-products are solids that can be used as a substrate-support matrix for fungal growth in solid fermentation. On the other hand, with the fermentation, the valorization of grapefruit by-products can be done because of recovery of secondary metabolites such as antioxidants. Fermentations were done using Raimbault columns as bioreactors. Two mixture levels (50 and 70% moisture content) were evaluated. Fungal growth was analysed online by CO2 quantification for fermentation of grapefruit by-products with Aspergillus niger GH1. Ethanolic extracts were used for antioxidant analyses (FRAP, LOI and DPPH·radical-scavenging). A. niger GH1 showed better growth on grapefruit by-products at 70% moisture content. Recovery of antioxidant compounds and solids reduction (50% of solids loss) was also higher at 70% moisture content. These results suggest that solid-state fermentation has great potential for valorization of grapefruit by-products as a support and a carbon source to produce antioxidants.