The undersea tremor struck at 08:39 am (0039 GMT) with its epicentre 62 kilometres (39 miles) southeast of Lanyu, an islet off Taiwan's southeastern coast, the country's Seismology Centre said.
A seismologist said the quake was felt only in Lanyu and in the town of Hengchun on the southernmost tip of the island.
Taiwan, which lies near the junction of two tectonic plates, is regularly shaken by earthquakes. The country's worst, a 7.6-magnitude quake, killed some 2,400 people in September 1999.