Abstract The present study aimed to examine the effects of solid-state fermentation (SSF) using selective A. niger strains on the amino acid, mineral, condensed tannin, and other chemical contents of olive leaves. The dried samples were divided into nonfermented (C) and fermented (F) olive leaves, and the latter were fermented by the following A. niger strains: ATCC® 9142™ (F1), ATCC® 200345™ (F2), ATCC® 52172™ (F3), and ATCC® 201572™ (F4), with three replicates for each treatment. Group F4 presented the best results, although all fermented groups generally presented higher performance than C. The total content of amino acids of the fermented olive leaves increased by 68–209% in comparison to that of C, while the cellulose content of the fermented olive leaves decreased by 7–25%. The ash, crude protein (CP), and ether extract (EE) contents increased after fermentation, but the crude fiber (CF) and nitrogen-free extract (NFE) contents decreased. The content of neutral detergent fiber (NDF) did not change, but acid detergent fiber (ADF) varied among the groups. The starch and sugar contents of all fermented groups except F1 also decreased compared to those of C. The mineral contents increased in all fermented groups, and the condensed tannin content varied according to the A. niger strain used. Thus, olive leaves fermented with different A. niger strains, especially F4, seem to have considerable potential as ruminant feed, as they are enriched with amino acids and minerals and have an improved chemical composition. However, these results should be supported and validated by animal experiments.