Brain amyloidosis is a key feature of Alzheimer’s disease (AD). It also incorporates cerebrovascular amyloid b (Aβ) in the form of cerebral amyloid angiopathy (CAA) involving neurovascular dysfunction. Studies recently showed by retrospective analysis that patients with mild cognitive impairment receiving a vasoactive drug cilostazol, a selective inhibitor of phosphodiesterase (PDE) III, exhibit significantly reduced cognitive decline. This study tested whether cilostazol protects against the disruption of the neurovascular unit and facilitates the arterial pulsation-driven perivascular drainage of Aβ in AD/CAA. Results showed that PDE III is abnormally upregulated in cerebral blood vessels of AD and CAA subjects and closely correlates with vascular amyloid burden. Cilostazol treatment maintained cerebral hyperemic and vasodilative responses to hypercapnia and acetylcholine, suppressed degeneration of pericytes and vascular smooth muscle cells, promoted perivascular drainage of soluble fluorescent Aβ1-40, and rescued cognitive deficits in Tg-SwDI mice. Although cilostazol decreased endogenous Aβ production in cultured neurons, C-terminal fragment of amyloid precursor protein expression was not altered in cilostazol-treated Tg-SwDI mice.The predominant action of cilostazol on Aβ metabolism is likely to facilitate Aβ clearance due to the sustained cerebrovascular function in vivo. These findings mechanistically demonstrate that cilostazol is a promising therapeutic approach for AD and CAA.