The US ship is heading to refuel the naval fleet in the region, including the Carl Vinson aircraft carrier group. The 249m-long Izumo can carry up to nine helicopters, and resembles US amphibious assault carriers.
The Japanese action was in response to North Korea threat to sink the Carl Vinson and a US submarine, amid rising tensions in the region. The Izumo's deployment follows recent joint exercises conducted by Japan and the USA, and other naval developments. China last week launched its second aircraft carrier.
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.
Japan's post-World War Two constitution bars its military from using force to resolve conflicts except in cases of self-defense.
Pacifist Japan has one of the most powerful militaries in the world, with a navy bigger and more modern than the British Royal Navy.
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 heading 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.
Tokyo could have sent a much smaller destroyer to escort the US Navy's Richard E Byrd. But sending the 27,000-tonne Izumo was perhaps too good an opportunity for Prime Minister Shinzo Abe and his Defence Minister Tomomi Inada to waste. Both Abe and Ms Inada are right-wing nationalists who want to scrap Japan's pacifist constitution. That is all but impossible. So instead, last year, Abe succeeded in pushing a new security law through parliament.
In effect the new law ignores the constitution, and says Japan's forces can come to the defense of its allies. Abe knows there is widespread public opposition to "remilitarizing" Japan. So in that context, the growing threat from North Korea is useful to him.
One view is that North Korea's threats are over exhausted and are just posturing and it is unlikely to follow through with an actual attack. Observers say that while North Korea is working towards achieving full nuclear missile capability, it is highly doubtful that it has a working long-range missile that could hit the USA. Several of its recent missile tests, including one earlier this month, have failed.
But if North Korea were to actually launch a strike, neighbouring South Korea and Japan could be top targets. Pyongyang appears to have working missiles that could hit those countries. Both countries have anti-missile defence systems - some supplied by the US - that could thwart incoming rockets. Japan's system employs Aegis destroyer ships and land-based Patriot PAC-3 units, according to its defence ministry. In South Korea, the US is in the midst of deploying its controversial Thaad anti-missile defence system. Both countries are also currently conducting pre-planned joint military exercises with the USA
Japanese authorities also issued guidelines last Friday to the public on how to survive a missile attack. They say it would only take minutes for a missile to reach Japan and urged citizens to seek shelter in buildings or underground. Citizens have been told that if a missile lands nearby, they should cover their mouths and noses and run away - if indoors they should stay away from windows to avoid injuries from shattering glass. They would be alerted about an incoming attack on TV, mobile phones, radio and outdoor loudspeaker systems via a system called J-Alert.
One prefecture government conducted an evacuation drill last month and several local officials are now calling for nationwide drills. Meanwhile in South Korea, which is more used to the North's threats, the mood appears to be less tense with little sign of ramped-up civil defence preparations. Authorities regularly conduct evacuation drills and also have an emergency alert system.
Due to its location just 56km (35 miles) from the North Korean border, the South Korean capital Seoul is also vulnerable to artillery fire. Last week, North Korea conducted a large-scale firing drill to mark its army's 85th founding anniversary. Analysts say the country has more than 20,000 artillery pieces and the BBC's Diplomatic Editor Mark Urban says its capability would be hard to neutralise.
South Korea meanwhile has a long history of border skirmishes with the North.
One of the most serious incidents in recent years took place in 2010, when North Korea shelled the island of Yeonpyeong at the two countries' maritime border, killing several soldiers and civilians. That same year saw the sinking of a South Korean navy ship in the same area, killing 46 sailors. The incident was attributed to a North Korean torpedo launched from a submarine. In 2015 Pyongyang also fired a rocket towards the South Korean town of Yeoncheon on the western border, prompting an evacuation.
Last month Pyongyang launched several missiles into the Sea of Japan, with three landing in Japanese waters. PM Shinzo Abe called it a "new stage of threat". Japanese authorities have also said that a North Korean long-range rocket launched in February 2016 passed over islands in Okinawa prefecture, travelling 1,600km (994 miles) within 10 minutes.
A French amphibious assault ship arrived in south-west Japan on Saturday for an exercise also involving Japanese, USA and British naval forces. South Korea has been conducting joint exercises with the USA as well.
Both the USA and North Korea have been trading heated rhetoric since the USA announced it would deploy a group of warships to the region. Pyongyang has reacted furiously and threatened a pre-emptive strike.
With tensions rising on the Korean peninsula, the possibility of a missile or nuclear weapon landing in South Korea and Japan, according to western propaganda mills, has now become more real.
Speculations are ripe to point that there is likely to be a nuclear or missile strike in region. However, such fears are manufactured by western media lords to terrorize the humanity and blame China and North Korea for all tensions in the region.
But no war can bring solution to the regional problems.