The monthly purchasing managers' index slipped to 51.4 last month from the prior month's 51.7, based on a 100-point scale on which numbers above 50 indicate expansion.
It was still the 12th straight month that factories reported expansion, according to the data compiled by the Federation of Logistics & Purchasing posted on China's official statistics website.
The survey found that new export orders dropped by a wider margin than overall new orders.
China's outsize manufacturing sector employs tens of millions of workers and the index is widely watched as an early indicator of the health of the economy, the world's second biggest.
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Earlier this month, official figures showed growth held steady at 6.9 per cent in the most recent quarter despite tighter lending controls aimed at taming surging debt levels.
Taken together, "today's data also hint at a slowdown in the broader economy," said Julian Evans-Pritchard of Capital Economics.
"We anticipate further weakness ahead as the crackdown on financial risks weighs on credit expansion and economic growth.