The Chicago purchasing managers' index declined to 54.4 in August from 54.7 in July, missing expectations for an increase to 54.9, according to MNI indicators.
A reading above the 50 mark indicates expansion, a reading below 50 indicates contraction.
The decline was driven by decreases by the production and new orders indexes.
The employment index rose in August.
"While the slowdown earlier in the year looks temporary, we're still some way below the strong growth rates seen towards the end of 2014," Chief Economist of MNI Indicators Philip Uglow said.