The Chicago Purchasing Managers' Business Barometer slid to 60.5 in September from 64.3 in August, the National Association of Purchasing Management-Chicago said Tuesday.
Production and new orders "slowed" while inventories and input prices grew.
The data is compiled on a seasonally adjusted basis. An index reading below 50 signals a slowing economy, while a level above 50 suggests expansion.
Economists polled by Thomson Reuters predicted a 62.0 reading for the index.
"With the latest fall, the Barometer ended Q3 at an average of 59.1, down from the weather boosted 63.7 rate seen in Q2, but still showing the US economy is growing at a healthy clip," according to the release.
