The remarks by Hua Chunying were in response to a speech last week by Japanese Prime Minister Shinzo Abe, the state-run Xinhua news agency reported.
Abe said Japan's sovereignty over the islands, known as the Senkaku Islands by Japan as the Diaoyu Islands by China and Taiwan, wasn't questioned from 1895-1971.
Hua said Abe's claims during the speech were "ridiculous."
"The Diaoyu Islands are an inherent part of China's territory," she said at a news briefing.
The spokeswoman said Japan "did not fulfill its due international obligations" after the end of World War II, by failing to return the islands to China.
"All Japanese activities regarding the Diaoyu Islands are based on illegal theft and occupation and are therefore illegal and invalid," she said.
The islands represent little in terms of land mass but control over the group involves fishing rights and ownership of possible natural resources, such as oil, under the seabed.