Further pressure came from expectations that US farmers will again expand soybean planting next year after private analytics firm raised its 2018 soy acreage forecast on Friday.
Benchmark CBOT January soybeans ended down 5-3/4 cents at $9.61-1/2 a bushel, the lowest since mid-September.
Soymeal and soyoil futures also closed lower.
Weekend rains in Argentina brought relief to parched farm belt and allowed planting to resume in most of the affected areas.
Brazilian farmers are expected to produce 114.57 million tonnes of soybeans in the 2017/18 crop cycle, consultancy Safras & Mercado said on Monday, cutting its output forecast for the oilseed.
The US Agriculture Department said private exporters sold 396,000 tonnes of US soybeans to China for 2017/18 delivery, the second large soy sales announcement in as many days.
The supplement to the CFTC's weekly commitments report showed large speculators flipped back to a net short position in CBOT soybeans in the week to Dec. 12, after holding a net long position for the previous two weeks.