The helicopter carrier Izumo is escorting a US supply vessel heading to refuel the naval fleet in the region.
The ships include the Carl Vinson group which was sent to the Korean peninsula.
North Korea has threatened a pre-emptive strike raising tensions in the region.
It also carried out a failed missile test on Sunday, despite repeated warnings from the US and others to stop its nuclear and missile activity.
The 249m long Izumo can carry up to nine helicopters, and resembles US amphibious assault carriers, reported The Japan Times.
Kyodo news agency said it would leave from its base in Yokosuka south of Tokyo to meet with the US supply ship, and accompany it to Shikoku in western Japan.
The Izumo is the first warship deployed under new laws passed in 2015, which allow Japan to come to the aid of an ally under attack.
Critics say this violates Japan's post-World War Two pacifist constitution and could lead it into unnecessary wars abroad.
Japan's constitution bars it from using force to resolve international conflicts except in cases of self-defence.
The Izumo's deployment follows recent joint military exercises conducted by Japan and the US, and other naval developments amid increased tensions.
A French amphibious assault ship arrived in southwest Japan on Saturday for an exercise involving Japanese, French, US and British naval forces. China last week also debuted its second large aircraft carrier.
South Korea has been conducting joint exercises with the US as well.