Corn and wheat also edged higher as a weaker dollar, which touched a new three-year against a basket of currencies, lent broad support to dollar-priced commodities.
The most-active soybean contract on the Chicago Board Of Trade was up 0.3 percent at $9.94-3/4 a bushel by 1220 GMT, the highest since Dec. 8 and approaching the psychological $10 threshold.
Soybeans extended gains to a ninth straight session, their longest rally since March 2012.
Persistent dryness in Argentina's main crop belt has worried traders and given impetus to a soybean market that had been weighed down by swelling supplies from top exporter Brazil.
"We have seen a big run-up in soybean prices as there have been weather concerns in Argentina," said Phin Ziebell, agribusiness economist at National Australia Bank.
"But the rally could be running out of steam as there is no major supply risk with ample supplies in the United States and Brazil."
Dryness there has fuelled forecasts of lost plantings and yield damage that could potentially tighten global supplies and make US exports look more attractive.
"More of the same in Argentina as dry weather stays entrenched over the core (grain belt)," Thomson Reuters Agriculture Research analysts said in a note.
CBOT corn was up 0.1 percent at $3.57 a bushel.
Weather worries in Argentina have also supported corn.
The US Department of Agriculture's (USDA) local office in Argentina on Wednesday projected the country's corn crop at 40 million tonnes this season, 2 million below the USDA's current official forecast.
The grains complex had been underpinned by the dollar's slide, fuelled by comments from US Treasury Secretary Steven Mnuchin welcoming a weaker currency.
CBOT wheat added 0.5 percent to $4.35 a bushel, trading at its highest levels in almost two weeks. It closed up 2.7 percent on Wednesday in a short-covering rally fuelled by the tumbling dollar.
Commodity funds were net buyers of Chicago Board of Trade corn, wheat and soybean futures contracts on Wednesday, traders said.