"Glorious Mission Online", China's answer to "Call of Duty", marks the 86th anniversary of the founding of the People's Liberation Army (PLA).
The game, an online version of an earlier first-person shooter used by the PLA to train troops, features the East China Sea islands known as Diaoyu by Beijing and Senkaku by Tokyo.
Tensions have been mounting over the islands, which are claimed by China but controlled by Japan. Beijing's vessels regularly sail into the disputed waters and, according to state media, tell Japanese ships they are encroaching on its territory.
Images from the game's website are labelled "Guard the Diaoyu islands", and a trailer posted online features shots of Japan's Prime Minister Shinzo Abe.
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It also shows planes taking off from a computer-generated version of China's first aircraft carrier, the Liaoning, which went into service last year.
The game was only due to become available at 5 pm (0900 GMT), but its designers Giant Interactive Group, who developed it jointly with the PLA, told AFP millions of users had already registered to play.
"Our relationship with the military is like the relationship between the US army and Hollywood," he said.
The release comes at a time of increased fears over the PLA's expansion amongst China's neighbours -- Beijing is also in dispute with several countries in the South China Sea.
But Gu linked the game with attempts by the PLA to present itself as more transparent, including inviting foreign media to tour military bases, to boost its image abroad.
"In Western games the People's Liberation Army is always the enemy, this is the first game where it is on the good side.