Plasma amyloid-β (Aβ) markers are significant predictors of Aβ pathology. However, their prognostic value for cognition in patients with Alzheimer's disease (AD) is unknown. We compared plasma amyloid-β precursor protein (APP)_669–711 and Aβ_1–42 levels between cognitively unimpaired participants (CU) and those with MCI due to AD and AD dementia. The CU group was divided into CU+ or CU- groups according to presence of Aβ pathology. All patients with AD exhibited Aβ pathology. The plasma APP_669–711/Aβ_1–42 ratio was significantly elevated in patients with CU+, MCI+, and AD+ compared with those with CU-. Furthermore, the plasma APP_669–711/Aβ_1–42 ratio was significantly correlated with the MMSE score (rs = −0.544, p < 0.001). Analysis of the Aβ+ group revealed that the significant relationship between MMSE score and plasma APP_669–711/Aβ_1–42 ratio remained unchanged (rs = −0.244, p = 0.027). Therefore, we conclude that the plasma APP_669–711/Aβ_1–42 ratio is associated with cognition in patients with AD.