The official Purchasing Managers' Index (PMI), which tracks activity in factories and workshops, fell to 49.4, figures from the National Bureau of Statistics (NBS) showed.
It was the lowest figure since 49.2 in August 2012, and was below the median forecast of 49.6 in a Bloomberg survey of economists.
PMI readings above 50 signal expanding activity, while anything below indicates shrinkage, and investors watch the figures closely as the first available indicators of the country's economic health each month.
"The manufacturing sector will likely face a tough year ahead on the back of overcapacity, weakening global demand, and government's plans to tackle pollution," ANZ economists Liu Ligang and Louis Lam said in a report.
China's economy, which is a vital driver of global expansion, grew 6.9 per cent last year, its weakest rate in a quarter of a century.
China's leaders -- who targeted growth of "about seven percent" -- are looking to transform the economy away from the investment and exports of the past to one more oriented towards domestic consumer demand, but the transition is proving bumpy, and the growth slowdown has alarmed investors worldwide.