Wheat also extended its rally, led by a 3pc jump for Kansas futures as the wet weather also raised the prospect of damage to maturing winter wheat plants.
Soybeans rose to recover some of Friday's sharp losses.
Reaction to US weather risks has been amplified by investment funds that have been holding large short positions in Chicago grains.
The most-active corn contract on the Chicago Board of Trade was up 1.6pc at $3.89-1/2 a bushel by 1045 GMT.
It earlier reached its highest since early June last tear at $3.89-3/4.
"The talk between traders is that 'prevented planted' acres for corn may be as high as 5 million acres," said Ole Houe, director of advisory services at brokerage IKON Commodities.
"Whereas in the past we have never seen it above 3.6 million acres, and then there is the debate over 'yield drag' from later-planted corn."
The western side of the US Midwest grain belt is forecast to receive heavy showers this week, compounding soggy field conditions after torrential rain and snow this spring.
The market is awaiting the US Department of Agriculture's next weekly crop update on Monday to see whether farmers made much headway in planting during relatively drier conditions last week.
In wheat, the most active Kansas hard red winter wheat futures were up 3.4pc at a one-month high of $4.34-1/2.
Chicago soft red winter wheat was up 1.6pc at $4.74-1/2 a bushel after touching a six-week high of $4.74-3/4.
As well as reacting to corn prices, the wheat market has been supported by weather risks for spring wheat planting and winter wheat development.
"It is not only the delays to planting that are playing their part, but also concerns about the quality of the young winter wheat as excessive moisture increases the risk of fungal infections," Commerzbank analysts said in a note.
CBOT soybeans added 1.2pc to $8.31-3/4 a bushel after dropping 2.1pc in the previous session.
The possibility of farmers switching acreage from corn to later-planted soybeans have weighed on the oilseed along with US-Chinese trade tensions.
However, doubts over soybean export demand created by the fractious US-China trade talks could tempt some farmers to stick with corn.