Japan's coast guard is now questioning the activists, who had earlier been denied permission to visit the islands.
Japan's controls the archipelago but it is also claimed by China and Taiwan.
A group of pro-China activists landed on the islands earlier this week, enraging Japanese nationalist groups.
The Japanese launched their flotilla on Saturday, saying they wanted to commemorate the Japanese who died near the islands in World War II.
The AFP news agency, which has a reporter on board one of the boats, reported that the activists were planning to climb to the highest point of the island and plant a Japanese flag.
China warned earlier that the expedition would undermine its territorial sovereignty.
The islands lie on a vital shipping lane, and are surrounded by deposits of oil and gas.
History of tension
The AFP journalist aboard one of the boats confirmed that the flotilla had arrived off the main island, Uotsurijima, at sunrise.
Earlier, Chinese foreign ministry spokesman Qin Gan said: "Any unilateral action taken by Japan on the Diaoyu Islands is illegal and invalid."
Earlier this week, pro-Chinese activists sailed to the disputed island chain from Hong Kong in a protest aimed at promoting Chinese sovereignty.
Chinese state media had praised Japan's "wise" decision to free them, saying that the speedy action had averted a deterioration in relations.
Rows over the disputed islands have caused Sino-Japanese ties to freeze in the past.
China claims the islands have been a part of its territory since ancient times, but Japan says it took control of the archipelago in the late 1890s after making sure they were uninhabited.
In September 2010, relations plummeted after the arrest of a Chinese trawler captain near the islands.
The captain was accused of ramming two Japanese patrol vessels in the area, but Japan eventually dropped the charges against him.
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