Obesity worsens inflammatory arthritis severity, even in non-load-bearing joints, but the mechanism is unknown. Here, we show that there is an immunological mechanism mediated by T cells in adipose tissue. Using an antigen-induced arthritis model with trackable, arthritis-inducing CD8+ OT-I T cells, we found that OT-I T cells home to visceral adipose tissue (VAT) and expand there in the obese high-fat diet (HFD) context. Transplant of VAT from arthritic mice increased arthritis severity in naïve recipient mice and was ameliorated by CD8 T cell depletion. Bulk RNA sequencing identified pro-inflammatory changes to OT-I T cells in VAT characterized by increased IFN α and γ signaling after HFD. Intraperitoneal injection of IFNα, but not IFNγ, expanded CD8 T cell numbers in VAT. HFD-induced expansion of VAT CD8 T cells was ameliorated with global Ifnar1 deletion, and importantly, genetic deletion of Ifnar1 in T cells decreased arthritis severity in obese mice. These results provide a mechanistic explanation of how obesity worsens autoimmunity.