Early diagnosis of late‐onset Alzheimer's disease (AD) by peripheral biomarkers remains a challenge; many have been proposed, but none have been evaluated in a prospective manner. CLUSTERIN (CLU), a chaperone protein expressed in the brain and found in relatively high concentrations in plasma, is a promising candidate. CLU contributes to the elimination of β‐amyloid (Aβ), which is associated to neurofibrillary tangles and to the genetic risk for AD. We performed a longitudinal measurement of CLU in the brain and the plasma in 3xTgAD mice. Assessment of CLU was also conducted in 12‐month‐old TgF344‐AD rats. In humans, brain CLU was measured in non‐demented and in AD subjects. The plasma CLU was longitudinally measured in four cohorts defined as healthy controls that remained stable, healthy controls that presented a cognitive decline between the two measures, mild cognitive impairment (MCI) that presented a cognitive decline between the two measures and AD. A validation cohort composed of 19 MCI was used and plasma CLU was measured before and after conversion in AD. Increases in CLU were measured in the hippocampus of 3xTgAD and TgF344‐AD animals in the absence of plasmatic changes. CLU is heterogeneously expressed in the hippocampus in non‐demented individuals and increased in AD. In the plasma, two CLU levels were measured: low in controls and MCI, and high in AD. To validate that the elevation in CLU is associated with conversion to AD, a replication study showed, in a second group MCI patients converting to AD in the follow‐up that CLU levels increased in 16/19 individuals. The increase in brain CLU occurs in AD models as in humans, and seems to precede plasma variations, which could make it an AD therapeutic target. Plasma CLU seems to be a promising marker of cognitive decline, and its association with AD may be a useful complementary diagnostic tool.