The sub would "perform its combat mission as one of [the] core underwater offensive means of the naval force" of North Korea, KCNA said.
But shortly after the announcement, military experts began expressing doubts about whether the submarine actually marked a significant breakthrough.
Joseph Dempsey, a defence analyst for the International Institute for Strategic Studies in London, said on X that it appeared to be a "modified" Romeo class Soviet submarine and that the photos could have been manipulated.
South Korea's General Staff in Seoul said it, along with the United States, has been monitoring the launch preparations.
An analysis of the external features shows that parts have been enlarged to carry rockets. But the submarine in its current form is "not capable of normal operation," the General Staff said.
Last month, Kim decried a recent announcement by "the gang bosses" of the US, Japan and South Korea that they would hold regular joint military exercises.
The North Korean ruler said the situation required his country's naval force to be strengthened.
UN resolutions prohibit North Korea from testing ballistic missiles of any range, some of which are capable of carrying a nuclear warhead.
Tests of cruise missiles are not subject to sanctions against Pyongyang. Such weapons, however, can also be used to deliver nuclear warheads.
