The exercise, called Foal Eagle, comes amid heightened tension following the latest test launch of a ballistic missile by the North on Feb. 12 and in the past prompted threats by Pyongyang to launch military action in retaliation.
South Korea's Defense Ministry and the US military based in the South confirmed the start of the drills on Wednesday that will continue until the end of April but did not immediately provide further details.
The exercise last year involved about 17,000 American troops and more than 300,000 South Koreans.
US Defense Secretary Jim Mattis spoke with South Korean Defense Minister Han Min-Koo early on Wednesday by telephone and said the United States remains steadfast in its commitment to the defense of its ally.
Mattis welcomed a deal signed by South Korea with the Lotte Group conglomerate this week to secure the land to station the Terminal High Altitude Area Defense (THAAD) missile defense system in the South, the two countries said, Reuters reported.
South Korea has said it and the United States aim to make the system, which the two countries decided last year to deploy in response to the North Korean missile threat, operational by the end of the year.
Han said in the phone call with Mattis that this year's joint drills will be conducted at a similar scale as last year's, which the South's Defense Ministry had called the "largest-ever" exercises by the allies.
North Korea's official KCNA news agency said earlier on Wednesday its leader Kim Jong Un inspected the headquarters of a major military unit and issued guidance on increasing combat readiness.