The Thomson Reuters/University of Michigan preliminary consumer sentiment index fell to 92.9 in August from a final reading of 93.1 in July, missing expectations for an increase to 93.5.
"Consumer confidence was virtually unchanged in early August from the July reading, marking its highest nine month average since 2004. Renewed strength in personal finances largely offset slight declines in prospects for the national economy and buying conditions," the Surveys of Consumers chief economist at the University of Michigan Richard Curtin said.
The decline in the index was mainly driven by falls in the index of current economic conditions and the index of consumer expectations.
The index of current economic conditions declined to 107.1 in August from 107.2 in July, while the index of consumer expectations decreased to 83.8 from 84.1.
The one-year inflation expectations remained unchanged at to 2.8% in August.