Wheat also gained 2.5 percent, tracking corn prices and building on a weather-driven rally in the US markets which has lifted corn 46 percent and wheat 35 percent in the past four weeks.
"Strong gains reflect the ongoing supply issues in the US as forecasts of high temperatures over the week will lead to further deterioration in corn and soybeans yield potential," said Luke Mathews, a commodities strategist at the Commonwealth Bank of Australia.
"Even though there are some forecasts of light rains, there is consensus out there that the rain will hardly be enough to offset the drought."
Chicago Board of Trade new-crop December corn rose as much as 4 percent to a contract high of $7.70 a bushel.
Actively traded November soybeans gained as much as 2.4 percent to $15.90-1/4 a bushel, a contract high, while September wheat added 2.5 percent to $8.68-3/4 a bushel.
China's Dalian soybeans and corn futures jumped, mirroring gains in US markets, as the world's biggest soybean importer and a key corn buyer takes a hit from rally in grain and oilseed prices.
Actively traded Dalian soybean futures were up 3.3 percent by 0402 GMT and corn gained 2.3 percent.
"China's demand for soybeans remains robust as the livestock breeding sector has entered peak consumption period, which will draw down the stocks at ports during August to October period," said Xia Tian, an analyst with Yong'an Futures, referring to the winter months and holiday period when meat consumption rises.
Last week, China's soybean stocks at ports fell for a second straight week to about 6.6 million tonnes from 6.7 million tonnes previous week.
SHRINKING CROPS
In the United States, a lack of meaningful rain in areas hardest hit by the worst drought in nearly 25 years is shrinking crops in the world's top corn and soybean exporter.
On Friday, forecasters said there was little relief in sight this week, with temperatures set to return to the high 90s to 100 degrees Fahrenheit in the western Corn Belt, and rise to the upper 90s F in the central-eastern Midwest.
Drought stress has already dragged corn and soy crop condition ratings to the lowest point for this time of year since 1988, and traders are expecting further downgrades in the US Department of Agriculture's weekly report on Monday.
The corn and soybeans crop were rated at a 25-year low, with just 40 percent in good-to-excellent conditions as of July 8.
After getting off to a record planting pace amid one of the mildest winters in decades, the corn crop ran into trouble when the weather turned dry and temperatures hit triple-digits.
Commodity funds were net buyers of an estimated 12,000 corn contracts on the day, along with a net 5,000 soybean contracts, trade sources said.
Large speculators raised their bullish bet on corn to a three-month high as investors jumped on a rally fueled by drought in key growing areas of the Midwest, regulatory data released on Friday showed.
The Commodity Futures Trading Commission's weekly commitments of traders report also showed that noncommercial traders, a category that includes hedge funds, trimmed their net long stake in soybeans as some speculators placed bets that the market had peaked.
The Midwest drought has done considerable damage to this year's corn crop. The USDA last week slashed its corn yield estimate for the world's top grower and exporter by an unprecedented 20 bushels, to 146 bushels per acre.