The quake, also felt in Tokyo, measured 3 on the Japanese intensity scale of 7, normally not enough to cause serious damage. Police in southeastern Hokkaido, Japan's northernmost main island, urged residents and fishermen not to get too close to coastal areas, TV news reports said.
The effects of the tsunami were expected to be felt between 2210 GMT and 2230 GMT.
The shallow quake struck an area about 310 km (190 miles) east of Japan's main island of Honshu at 2139 GMT.
Japan is one of the world's most earthquake-prone countries, with a tremor occurring at least every five minutes.