The Conference Board said its consumer confidence index declined from 68.7 in April to 64.9 in May.
"Consumers were less positive about current business and labor market conditions, and they were more pessimistic about the short-term outlook," said the board's Lynn Franco.
"However, consumers were more upbeat about their income prospects, which should help sustain spending."
The data, which are keenly watched on Wall Street, could point to slower growth ahead.
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