The municipal court in Shijizhuang in northern China found Lyu Yueting guilty of adding dangerous substances to food in 2008, the official Xinhua News Agency reported, adding the dumplings also had sickened four Chinese.
Lyu can appeal the ruling.
Japan's Kyodo News reported the dumpling poisoning case surfaced six years ago prior to the Beijing Olympics, raising concerns over food imports from China. The case also strained relations between the two countries, which have since worsened with the breakout of a bitter territorial dispute over the Japanese-controlled Senkaku Islands in the East China Sea.
The Kyodo report said the 39-year-old Lyu had admitted last year to poisoning the frozen "gyoza" dumplings. The report said the food item had sickened 10 people in Japan, including a 5-year-old girl and four Chinese.
Lyu had been a temporary worker at a food plant in Hebei province where the dumplings were made and later exported to Japan, Kyodo said. The incident caused the food plant to close down.
Kyodo quoted prosecutors the accused injected insecticide into the into the dumplings due to frustration over his wages and those of his colleagues. The report said the incident had nothing to do with any resentment against Japan.