Tokyo says the fleet included six coastguard ships, three of which appeared to be armed.
Japan has summoned Chinese diplomats to protest.
Over recent years, Beijing has become increasingly assertive about its rights in waters it believes are Chinese.
Testing Japan's resolve
In this latest incident, Chinese boats sailed close to the Senkaku Islands, which are administered by Japan but also claimed by China. Beijing calls them the Diaoyu Islands.
A Japanese foreign ministry spokesman said the fleet's despatch was a unilateral escalation of tension in the area.
Less than a decade ago, Tokyo and Beijing talked of jointly exploiting the resources of the East China Sea, the waterway that separates the two countries.
But since then tension has increased, particularly over the disputed Senkakus, which are uninhabited.
Over recent years, China has sent an increasing number of ships towards the islands, in what appears to be an attempt to test Japan's resolve to defend them.
Beijing has also become increasingly assertive about its territorial claims in the South China Sea.
But last month a tribunal under the Permanent Court of Arbitration in the Hague ruled that most of Beijing's claims in that sea had no legal basis.
China said it would ignore the decision.