Submarine No. 841 - named Hero Kim Kun Ok after a North Korean historical figure - will be one of the main "underwater offensive means of the naval force" of North Korea, leader Kim Jong-un said at the launch ceremony on Wednesday.
Analysts said the vessel appears to be a modified Soviet-era Romeo-class submarine, which North Korea acquired from China in the 1970s and began producing domestically.
Its design, with 10 launch tube hatches, showed it was most likely armed with ballistic missiles and cruise missiles, analysts said.
But such weapons won't add much value to the North's more robust land-based nuclear forces, because its submarines might not survive as long during a war, said Vann Van Diepen, a former United States government weapons expert who works with the 38 North project in Washington.
"When this thing is field deployed, it's going to be quite vulnerable to allied anti-submarine warfare," he said.
"So I think from a sort of hard-headed military standpoint this does not make a lot of sense."
South Korea's military said the submarine did not appear ready for normal operations, and there were signs North Korea was attempting to exaggerate its capabilities.
At the launch ceremony, Kim said arming the navy with nuclear weapons was an urgent task and promised more underwater and surface vessels equipped with tactical nuclear weapons for the naval forces, news agency KCNA reported.
"The submarine-launching ceremony heralded the beginning of a new chapter for bolstering up the naval force of the DPRK," KCNA said, using the initials of the North's official name, the Democratic People's Republic of Korea.
North Korea plans to turn other existing submarines into nuclear armed vessels, and accelerate its push to eventually build nuclear-powered submarines, Kim said.
"Achieving a rapid development of our naval forces ... is a priority that cannot be delayed given ... the enemies' recent aggressive moves and military acts," he said in a speech, apparently referring to the US and South Korea.
North Korea's nuclear weapons and ballistic missile programs are banned by United Nations Security Council resolutions, and the submarine launch drew condemnation from South Korea and Japan.
"North Korea's military activity is posing graver and more imminent threat to our country's security than before," Japanese Chief Cabinet Secretary Hirokazu Matsuno told a briefing.
The designation as a "tactical" submarine suggests it does not carry submarine-launched ballistic missiles (SLBM) that can reach the US mainland, but rather smaller, short-range SLBMs or submarine-launched cruise missiles (SLCM) capable of striking South Korea, Japan, or other regional targets, said Choi Il, a retired South Korean submarine captain.
The rear of the submarine's sail - the tower that juts out of the top of the hull - was expanded and 10 vertical launch tubes, four large and six small, were installed, likely for SLBMs and SLCMs, he said.
North Korea has test-fired both SLBMs and SLCMs.
The launching ceremony comes as North Korea is set to mark the 75th anniversary of its founding day on Saturday and follows reports Kim plans to travel to Russia this month to meet President Vladimir Putin to discuss weapons supplies to Moscow.
South Korean President Yoon Suk Yeol on Thursday met with Chinese Premier Li Qiang in Jakarta, and asked Beijing to do more as a UN Security Council member to address North Korea's nuclear threat.
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