First reports said tremors were felt in Patong, the west coast holiday hub, and at sports facilities in Saphan Hin, a large public park in Phuket City, on the island's east coast.
The National Disaster Warning Centre said a seaquake at Koh Yao at 1.02pm was at two kilometres below the surface.
The quake was also felt in Phang Nga, the holiday province north of Phuket. Later reports said it was also felt in Krabi, another Andaman holiday province.
One reader said the shake was felt at Phuket's central Heroine's Monument for six seconds.
Another reader based in southern Phuket at Chalong said: ''I felt a light shaking that lasted maybe 10-15 seconds . . . not sure at first if it was an earthquake or maybe a passing large vehicle but quite noticeable.''
There were no reports of a tsunami. The epicentre of the quake was reported to be at Latitude 7.87 Longitude 98.57.
''We were on the beach at Nai Yang [near Phuket airport] and felt it there,'' said one caller to Phuketwan. ''But there was no panic.''
People on Koh Yao Yai felt the tremors but others on the smaller neighboring island of Koh Yao Noi said they felt nothing.
Japanese tourists on a day-trip atoll at Koh Kai asked to be taken back to Phuket before their time was up.
A tsunami that followed an earthquake off Indonesia at magnitude 9.2 on December 26, 2004, killed 5400 people in Thailand and more than 220,000 throughout the Indian Ocean region.
A 4.3 magnitude quake struck Phuket at 4.44pm on April 16, 2012 with its epicenter at Phuket's central Heroine's Monument Thalang district while another quake, at 5.4-magnitude, struck Indonesia's Sumatra two minutes later.
That earthquake came just five days after a tsunami alert led thousands of people to evacuate Patong in what proved to be a false alarm.