It is the first time that Lee has visited the island of Yeonpyeong near the Yellow Sea border with the North. The island was bombarded in the North's shelling attack in November 2010, leaving two South Korean Marines and two civilians dead.
The visit also came amid allegations that Lee's liberal predecessor, late President Roh Moo-hyun, said during his 2007 summit with then North Korean leader Kim Jong-il that Seoul would not insist on the sea border, known as the Northern Limit Line (NLL), which Pyongyang does not recognize.
"Our military should safeguard the NLL by staking their lives until unification comes. That is the very way to safeguard peace," said Lee, dressed in a fatigue jacket, during a lunch meeting with Marines and residents on the island, located just 1.5 km from the sea border.
"Safeguarding the line well contributes to maintaining peace. In addition, it is to protect the lives and safety of our people. We have to firmly defend the NLL. The government has such a strong and firm stance," he said.
North Korea has never recognized the NLL, which was drawn unilaterally by the U.S.-led United Nations Command when the 1950-53 Korean War ended, and demands that the line be drawn farther south.
Areas near the border have been the scene of a number of bloody inter-Korean clashes. The two sides fought naval gun-battles in the area in 1999, 2002 and 2009. The North also torpedoed and sank a South Korean warship in the area in March 2010, killing 46 sailors.
Officials said the visit was aimed at checking on military vigilance in the front-line area.
Concerns about border security have risen following revelations that the military was unaware that a North Korean soldier had slipped past the eastern section of the heavily armed land border until he knocked at the barracks door of a front-line unit in a defection bid early this month.
Last week, Lee called in Defense Minister Kim Kwan-jin and issued a stern rebuke for lax military discipline. On Monday, the defense chief offered a public apology and said 14 senior Army officers will be reprimanded, including five general-grade officers.
"One North Korean soldier knocked on the door and said he wanted to defect to South Korea," Lee said during Thursday's island visit. "Our people are concerned about what would have happened if that North Korean soldier was armed?"
Lee also said South Korea should mount a powerful counterstrike if North Korea ever again launches an attack like the 2010 shelling, but he also stressed that the country's goal is not to win a war, but to deter provocations.