An official Chinese news portal unveiled an online game on Thursday that allows users to shoot Japanese war criminals, in a move that is likely to please Chinese nationalists but do little to ease tensions between Beijing and Tokyo.

The move came the same day that China’s top legislative body, the Standing Committee of the National People’s Congress, ratified two new national days: Sept. 3 to mark Japan’s defeat in World War II and Dec. 13 to commemorate the victims of the 1937 Nanjing Massacre, when the Japanese military went on a deadly six-week rampage in the city.

The game launched by People’s Daily Online Weibo, a microblog affiliated with the newspaper of the ruling Communist Party, is provocatively titled Shoot the Devils. In China, “Japanese devil” is a common slur. The game also features an offensive caricature of a Japanese soldier....