The latest data was worse than economists had expected and unsettled global financial markets. Uncertainty about the extent of China's slowdown has been on the radar of investors, particularly after the Federal Reserve mentioned China as one of its reasons for not raising interest rates last week.
The preliminary Caixin/Markit index, which is based on a survey of factory purchasing managers, fell to 47.0 in September from 47.3 in August. Numbers below 50 on the 100-point index indicate contraction.
China's factories cut output, staff numbers and prices at a faster pace as both new export orders and overall new orders fell, the report said. Chinese manufacturers employ many millions of workers and represent a big part of the overall economy but they are feeling the effects of a weak recovery in major markets overseas.
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The data will add to the pressure on China's communist leaders as they try to prevent growth from falling too sharply.
China's economic growth held steady at 7 percent in the most recent quarter, which was its weakest performance since 2008.
Growth has fallen from the double-digit rates of the previous decade as Beijing tries to wean the economy off its reliance on trade and investment in favor of domestic spending on goods and services but the transition is challenging.
Policymakers in Beijing have cut interest rates five times since November and also slashed bank reserve requirement ratios in a bid to bolster economic growth.