Gen. Curtis "Mike" Scaparrotti made the comment as he assumed command of the 28,000 U.S. forces during a ceremony in Seoul, Stars and Stripes reported.
A veteran of Iraq and Afghanistan, Scaparrotti assumed command from Gen. James Thurman, who is retiring after more than 38 years in the military.
Scaparrotti vowed to remain "vigilant and steady," adding "I am confident in our war fighting abilities and our ability to deter North Korean aggression and, if necessary, to decisively fight and win."
The change of command comes amidst discussions by Washington and Seoul about whether to delay the transfer of control to South Korean forces in the event of war. That control is currently scheduled to shift in December 2015.
Scaparrotti will oversee a $10.7 billion project to move U.S. troops from north of Seoul to southern parts of the country.
He also will lead the Combined Forces Command and the United Nations Command.