Japan's Coast Guard said on Sunday that two Chinese vessels entered what Japan considers its territorial waters near disputed islands in the East China Sea.
It marked the 21st time this year that Chinese boats have entered Japanese waters and the first since late August, a Japanese Coast Guard representative said.
Japan's government has complained of "relentless" intrusions in waters around the group of islets claimed by both nations. The contested territory contains islands known as the Senkaku in Japan and the Diaoyu in China.
Two Chinese Coast Guard ships entered the waters at about 10:45 a.m. (0145 GMT) on Sunday and approached a Japanese fishing boat, the Kyodo news agency said. The Japanese Coast Guard deployed patrol boats to the area, Kyodo said.
China's Chinese foreign ministry did not immediately respond to a Reuters request for comment. The defence ministry could not immediately be reached.