WASHINGTON—Japan's defense minister said Friday that Tokyo would "respond firmly" if China "disrupts the order" in waters contested by the two countries, taking a tough line as his country loosens long-standing restrictions on its military.

Japanese Defense Minister Itsunori Onodera, speaking at the nonpartisan Center for Strategic and International Studies in Washington, stressed that Japan isn't seeking a confrontation with China, citing efforts to make a hotline between Tokyo and Beijing operational and...