In a head-to-head comparison study including nine cohorts and 1,474 individuals, we show that plasma p-tau217 and medial temporal lobe tau-PET signal display similar associations with cognitive decline on a global cognitive composite test (R2PET = 0.34 versus R2plasma = 0.33, Pdifference = 0.653) and with progression to mild cognitive impairment (hazard ratio (HR)PET = 1.61 (1.48–1.76) versus HRplasma = 1.57 (1.43–1.72), Pdifference = 0.322). Here, MAPT is linked to Cognitive impairment.