The helicopter carrier Izumo is escorting a US supply vessel within Japanese waters.
The US ship is heading to refuel the naval fleet in the region, including the Carl Vinson aircraft carrier group.
North Korea has threatened to sink the Carl Vinson and a US submarine, amid rising 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 BBC's Rupert Wingfield-Hayes in Tokyo says the Izumo is the pride of the Japanese navy, and is by far its biggest ship.
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 was leaving its base in Yokosuka south of Tokyo to join the US supply ship, and accompany it to waters off Shikoku in western Japan.
Under Prime Minister Shinzo Abe, Japan is gradually pushing the limits of what its modern and powerful military is allowed to do, and with tensions high on the Korean peninsula, Mr Abe appears keen to try out the new laws for the first time, our correspondent says.
Japan's post-World War Two constitution bars its military from using force to resolve conflicts except in cases of self-defence.
The Izumo is the first warship deployed outside of military exercises under new laws passed in 2015 that allow Japan to come to the aid of an ally under attack known as "collective self-defence".
Mr Abe's government, which pushed for the change, faced criticism that the new laws could lead the country into unnecessary wars abroad - something Mr Abe has rejected.
When is military action allowed?
- Japan can protect the weapons and equipment of its allies' armed forces who are defending Japan
- It can provide logistical support to allies involved in situations with "important influence" on Japan's security - for example it could support South Korea if the North invaded, but may stop short of sending troops as this may be unconstitutional
- Japan can shoot down a North Korean missile headed for the US
- Military action such as minesweeping to keep shipping lanes secure, even in an active conflict zone, may be allowed if the restriction on shipping threatened Japan's survival
The Izumo's deployment follows recent joint exercises conducted by Japan and the US, and other naval developments.
A French amphibious assault ship arrived in south-west Japan on Saturday for an exercise also involving Japanese, US and British naval forces. South Korea has been conducting joint exercises with the US as well.
China last week also launched its second aircraft carrier.