The Thomson Reuters/University of Michigan preliminary consumer sentiment index dropped to 88.6 in May from a final reading of 95.9 in April, missing expectations for an increase to 96.0. It was the lowest level since October 2014.
The Surveys of Consumers chief economist at the University of Michigan Richard Curtin said that the decline was a result of lower optimism regarding "quick and robust rebound" in the U.S. economy.
Mr Curtin added that consumers remained optimistic about their future personal finances.
A gauge of consumers' current economic conditions dropped to 103.0 in March from 106.9 in February.
The index of consumer expectations declined to 81.5 from 88.8.
The one-year inflation expectations in May climbed to 2.9% from 2.6% in April.