The world's largest currency hoard shrank by USD 99.5 billion in January to some USD 3.2 trillion, the People's Bank of China said on its website, the lowest since May 2012.
Worries about China's economy have pushed the yuan to a five-year low. The country saw its first-ever annual decline in foreign exchange reserves last year as Beijing tried to prevent a more drastic devaluation.
The PBoC is selling dollars to buy yuan amid a capital flight spurred by the slowing growth in the world's second largest economy.
"While the remaining reserves represent a substantial war chest, the rapid pace of depletion in recent months is simply unsustainable," IHS Global Insight economist Rajiv Biswas told Bloomberg News.
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Outflows increased "as expectations mount that the PBoC will eventually be forced to capitulate once its reserves are sufficiently depleted", he added.
George Magnus, economic commentator and associate at Oxford University's China Centre, wrote on Twitter: "China's fx reserves fell another USD 100bn... Clearly this can't go on for long."
China has also tightened some capital controls to try to curb outflows.
"The smaller decline in the reserves suggests that some capital outflow restrictions imposed in January worked," Shen Jianguang, chief Asia economist at Mizuho Securities, wrote in a note.
The drop in February will be much smaller, he added.
China has set its growth target for this year at between 6.5-7 per cent, the top economic planner said Wednesday, an acknowledgement that expansion -- already at its slowest pace in 25 years -- will continue to weaken.