Yonhap news agency, quoting a military official, says four soldiers are wounded. The reports could not be immediately verified.
South Korea's military returned fire, Yonhap said, citing Col. Lee Bung-woo, spokesman for the South's Joint Chiefs of Staff.
South Korea has deployed fighter jets to the island, Yonhap news agency said.
"A North Korean artillery unit staged an illegal firing provocation at 1434 PM (0534 GMT) and South Korean troops fired back immediately in self-defence," a defence ministry spokesman told AFP.
YTN said between 1,200 and 1,300 people lived on the island, citing a resident. TV pictures showed black and white smoke rising from the island.
South Korean military officials said that most shells hit a military base on the island. The South Korean army is on its highest peacetime alert.
North Korea's firing comes as South Korea conducts a military drill on the western coast. The "Hoguk Exercise," one of South Korea's three major annual defense exercises, began Monday with some 70,000 troops participating as well as US forces.
North Korea's recent claims that it has a new uranium enrichment facility has also raised tensions.
The countries' western maritime boundary has long been a flashpoint between the two Koreas.
The North does not recognise the border that was unilaterally drawn by the United Nations at the close of the 1950-53 Korean War.
North and South Korea have fought three bloody skirmishes near the maritime border in recent years, most recently in November 2009.