The North's official website -- Uriminzokkiri -- branded the legislators "a shameless bunch" and urged all Koreans to form a "unified force" against Japan's claim over the disputed Dokdo islands.
Seoul and Tokyo have been locked in a decades-old dispute over the islands known as Takeshima in Japan. South Korea controls the islands in the Sea of Japan (East Sea).
Four lawmakers from Japan's conservative opposition Liberal Democratic Party last week announced plans to visit Ulleung island -- the closest South Korean territory to the islets -- next month.
The latest diplomatic flare-up came after Seoul-based Korean Air operated a demonstration flight of its new Airbus 380 aircraft in June over Dokdo. In response Tokyo ordered its civil servants not to use Korean Air for one month.
This week South Korean prime minister Kim Hwang-Sik called the airline boycott a breach of international rules and also expressed regret over the Japanese lawmakers' planned visit.
"Where in the world is there such a shameless bunch?" the North's website said in a scathing editorial.
"All Koreans, with a unified force, should squash the attempt to steal Dokdo so that Japanese reactionaries would never set their eyes on this land," it said.
Cross-border relations have been at the lowest ebb in years since Seoul accused Pyongyang of torpedoing its warship in March last year at a loss of 46 lives.
But North Korea also angrily denounces Japan, which colonised the peninsula from 1910-1945, over atrocities committed during that period and its ongoing territorial rows with South Korea.