Dry reforming of methane (DRM) is a promising route for the resource utilization of greenhouse gases; however, catalyst deactivation due to carbon deposition remains a major challenge. In this work, Ru-based catalysts supported on rare-earth high-entropy oxide (HEO, LaPrSmEuNdO_x), medium-entropy oxide (MEO, LaPrSmO_x), and single-component La₂O₃ were synthesized to investigate the role of configurational entropy of oxide support in catalytic performance and mechanism towards DRM reaction. Ru/HEO, featuring a fluorite structure and highly dispersed Ru sites, demonstrated superior activity, H₂/CO ratio, and 50-h coke-resistant stability at 650 °C compared to Ru/MEO and Ru/La₂O₃. Catalytic studies, temperature-programmed characterizations, and spectral evidence revealed that Ru/HEO selectively activates CH₄ via partial dissociation to CH_x* and facilitates CO₂ activation by proper surface basicity and enhanced oxygen mobility and oxygen vacancy density. The in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectra identified the key reaction intermediates, confirming a cooperative conversion pathway of CH₄ and CO₂ on Ru/HEO for boosted activity and stability. This work provides insights into the entropy effect-driven structure design of active and anti-coking metal catalysts and mechanistic understanding of DRM reaction pathways.