The Conference Board said its consumer confidence index plunged 6.7 points this month to 81.9, the lowest level since 2014.
Economists polled by Reuters had forecast the index at 89.2.
"Consumer appraisals of current business conditions became negative for the first time since September 2024," said Dana Peterson, chief economist at the Conference Board.
"Perceptions of the current labour market also worsened, though remained within positive territory."
Write-in responses to the board's survey, collected from September 1-23, were mostly pessimistic this month, with frequent references to the high cost of petrol, goods and services.
Earlier this month, the US government reported that consumer inflation accelerated last month and petrol prices spiked as fighting in the Middle East dragged on.
The consumer price index rose 3.4 per cent last month compared with a year ago, the Labor Department said on Friday, just like July.
But inflation quickened month to month as costs jumped 0.4 per cent from July, quadrupling the 0.1 per cent registered in the previous month.
The US central bank two weeks ago raised its benchmark interest rate for the first time since 2023 in an effort to quell stubbornly high inflation, and the Federal Reserve signalled another rate hike could occur later this year.
with AP
