The Institute for Supply Management-Chicago business barometer fell to 49.7 from 53.0 in August. Economists had forecast an unchanged reading of 53.
A reading below 50 indicates contraction in the regional economy.
The forward-looking new orders index plummeted to 47.4, from 54.8. while the gauge of employment sank to 52.0 from 57.1 last month.
Following the report, stocks added to their losses on the final trading day of the third quarter and amid uncertainty ahead of the result of stress tests on Spanish banks.