The board's index slid to 99.8 from 105.6 in August, leaving it at its lowest mark since November 2005's 98.3.
Consumers' assessments of present-day conditions were also lower, dragging this index down to 121.7 in September from 130.1 in August.
The index measuring expectations for business conditions over the next six months also fell, to 85.2 in September from August's 89.2.
The confidence survey is based on polling of 5,000 U.S. households.
The cutoff date for the survey was Sept. 18, the day the Federal Reserve cut interest rates by surprising half-percentage point in an effort to rescue a sagging economy and restore confidence to shaky financial markets.
"Weaker business conditions combined with a less favorable job market continue to cast a cloud over consumers and heighten their sense of uncertainty and concern," said Lynn Franco, director of the Conference Board Consumer Research Center. "Looking ahead, little economic improvement is expected and with the holiday season right around the corner, this is not welcome news."
-- Dan Molinski, Dow Jones Newswires