We　report　on　a　giant　enhancement　in　the　thermal　responsivity　of　doubly　clamped　GaAs　microelectromechanical-system-　(MEMS)　beam　resonators　by　using　the　internal-mode-coupling　effect.　This　is　achieved　by　coupling　the　fundamental　bending　mode　with　the　fundamental　torsional　mode　of　the　MEMS-beam　resonators　through　the　cubic　Duffing　nonlinearity.　In　the　mode-coupling　regime,　we　find　that　when　the　input　heat　to　the　MEMS　resonators　is　modulated　at　a　particular　frequency,　the　resonance-frequency　shift　caused　by　heating　can　be　enhanced　by　almost　2　orders　of　magnitude.　The　observed　effect　is　promising　for　realization　of　high-sensitivity　thermal　sensing　by　use　of　MEMS　resonators,　such　as　ultrasensitive　terahertz　detection　at　room　temperature.　©　2020　American　Physical　Society.