Holistic healthcare practitioners have now started to focus on specific traditional medicinal mushrooms to treat rheumatoid arthritis (RA). Ganoderma lucidum (GL) is one of the oldest mushrooms that have been used in ancient Chinese medicine to treat inflammatory ailments, including autoimmune diseases such as RA. Spores from this mushroom have specific effects on immunomodulation, aging, and cancer. However, the effect of G. lucidum spores (GLS) on arthritis remains unclear. Therefore, we investigated the effects of GLS oil in a collagen-induced rheumatoid arthritis (CIA) model. Metabolomics analysis revealed that GLS oil contains ten acids, of which oleic acid (52.12%) and linoleic acid (16.77%) predominated. The GLS oil-treated CIA mice had a significantly lower clinical score (p = 0.0384) for RA than the control CIA mice. Moreover, GLS oil reduced CIA-induced cartilage degeneration and synovial membrane inflammation in the knee. The GLS oil group showed significantly reduced knee eosinophilia (p = 0.0056). Immunostaining of neutrophils revealed that neutrophils infiltrated the CIA group; however, infiltrated neutrophils were significantly reduced in the GLS oil group in both the knees (p = 0.0006) and ankles (p = 0.0023). GLS oil treatment substantially suppressed LPS- or TNF-α–induced IL-6 mRNA expression in primary cultured chondrocytes. IL-6 immunohistochemistry results showed that the protein levels of IL-6 were attenuated in the GLS oil group compared to the CIA group. These findings suggest that GLS oil may be useful for the development of RA drugs. Further clinical research is required to identify significant improvements.