North Korea conducted its sixth (and most recent to date) nuclear test on 3 September 2017, stating it had tested a thermonuclear weapon (hydrogen bomb). The test took place at a high point of the 2017–2018 North Korea crisis.

The United States Geological Survey reported an earthquake of 6.3 magnitude not far from North Korea's Punggye-ri nuclear test site. South Korean authorities said the earthquake seemed to be artificial, consistent with an underground nuclear test. The USGS, as well as China Earthquake Networks Center, reported that the initial event was followed by a second, smaller, earthquake at the site, several minutes later, which was characterized as a collapse of the cavity formed by the initial detonation.

While early intelligence and academic reports suggested yields between 100 and 160 kilotons of TNT, later yield estimates were between 250 and 400 kilotons. North Korea had previously claimed it tested a hydrogen bomb in January 2016, but this was widely discounted by experts. Analysts were more favorable to a true thermonuclear weapon explanation for the 2017 test, while yield estimates reach the upper limits of the largest pure fission or largest boosted fission weapons ever tested.

The test was internationally condemned by countries throughout Asia, as well as the five permanent members of the United Nations Security Council. Japan and South Korea announced new missile defense measures, with South Korea seeking Terminal High Altitude Area Defense batteries from the United States. In November, the United States relisted North Korea as a State Sponsor of Terrorism.

In April 2018, North Korea announced a unilateral nuclear and intercontinental ballistic missile testing moratorium. From 2021 the country extensively tested cruise missiles and short-range ballistic missiles, and in 2023 resumed ICBM tests, while as of 2025^[update] this remains the most recent confirmed nuclear test in the world.

The North Korean government announced that it had detonated a hydrogen (thermonuclear) bomb that could be loaded onto an intercontinental ballistic missile (ICBM). The announcement stated the warhead had a variable yield "the explosive power of which is adjustable from tens kiloton to hundreds kiloton (sic) ... [and] which can be detonated even at high altitudes for super-powerful EMP attack". A later technical announcement called the device a "two-stage thermo-nuclear weapon" and stated experimental measurements were fully compatible with the design specification, and there had been no leakage of radioactive materials from the underground nuclear test.

Photographs of North Korean leader Kim Jong Un inspecting a device resembling a thermonuclear weapon warhead were released a few hours before the test.

Analysts have tended to give credence to North Korea's claim that it was a hydrogen bomb. 38 North made a revised estimate for the test yield at 250 kT, making it near the maximum-containable yield for the Punggye-ri test site. Tom Plant, director of proliferation and nuclear policy at the Royal United Services Institute said, "The North Koreans do bluff sometimes, but when they make a concrete claim about their nuclear programme, more often than not it turns out to be true. ... I think the balance is in favour of it being a thermonuclear bomb rather than a conventional atom bomb."