The annual "Key Resolve" and "Foal Eagle" exercises -- condemned by North Korea as a rehearsal for invasion -- will be held from February 24-April 18, the allies' combined forces command announced in a statement.
A combined total of 12,700 US forces will participate in the two drills, the statement said, adding that North Korea had been informed of the dates and the "non-provocative" nature of the manoeuvres.
North Korea has repeatedly called on Seoul and Washington to scrap this year's exercises, warning at one point of an "unimaginable holocaust" if they went ahead.
The start of the drills will overlap with a reunion for families separated by the 1950-53 Korean War that is scheduled to be held February 20-25 at the North's Mount Kumgang resort.
Seoul and Pyongyang reached an agreement on the reunion last Wednesday, but only a day later the North threatened to pull out, citing US bomber sorties and "slanderous" reports in the South Korean press.
South Korean President Park Geun-Hye has urged Pyongyang to honour the agreement for the sake of the family members, many of whom are in advanced old age and frail health.
North Korea had cancelled a reunion event in September at the last minute and many expect it will do the same again, citing South Korea's insistence on holding the joint military exercises.
"Key Resolve" lasts just over a week and is a largely computer-simulated exercise, while the eight-week "Foal Eagle" drill involves air, ground and naval field training.
Seoul and Washington insist they are both defensive in nature, playing out various North Korean invasion scenarios.
"The scenarios are realistic, enabling us to train on our essential tasks and respond to any crisis which may arise," said General Curtis Scaparrotti, Combined Forces Command commander.
Last year's drills fuelled an unusually sharp and protracted surge in military tensions, with Pyongyang threatening a pre-emptive nuclear strike, and nuclear capable US stealth bombers making dummy runs over the Korean peninsula.
US defence official have indicated -- in an apparent effort to mollify the North -- that this year's drills will be slightly toned down, with no aircraft carrier and no strategic bombers.