Details remain sketchy but at least one tourist and the guide have been admitted to a hospital on Phuket. Tourists from other countries were on board.
The crash will highten concern about tourists' safety at sea in Thailand with two Chinese tourists killed recently in a crash off another Thai holiday destination, Pattaya.
Phuket Marine Police were alerted to the crash about 2.30pm by Suriya Tamchoo, the owner of a speedboat.
The Nontasak 9030, carrying 31 tourists and three guides, had crashed off Koh Kai Nok, a small atoll popular with day-trippers, officials were told.
The speedboat, reportedly returning from an outing to another larger island, Phi Phi in the neighboring province of Krabi, had crashed into the Sasiyanon 8 at the atoll's beach.
Zheng Dazhi, 36, injured an arm, Xu Yiming, 51, suffered a back injury, Zang Zoufa, 53, damaged an arm, and Zun Jiuxing, 42, broke ribs and required surgery at the Phuket Hospital.
A guide, Veeranan Srisuwan, 40, who suffered head wounds, was also admitted to Mission Hospital in Phuket City where the Chinese were treated for their injuries.
The two ''captains'' of both speedboats were taken to a small police station on the nearby island of Koh Yao Yai for questioning.
The men were named as Prayoon Kembase, who was at the wheel of Sasiyanon 8, and Sangnarin Sungkao, who was at the wheel of Nontasak 9030.
A Chinese tourist, Kang Xiao Xue, 24, died of injuries sustained in a speedboat crash off the same atoll in October last year when the ''captain'' of one speedboat slung four tourists being towed on an inflated banana float into the side of a second speedboat.
On June 19 this year, Chen Peng, 36, was killed on a Phuket holiday when struck by a propeller in the sea on a snorkelling trip off Phi Phi.
Earlier this month, Shen Baocheng, 27, narrowly escaped death when the jet-ski he was riding off Phuket's Patong beach crashed with a speedboat towing a parasailer.
The behavior of Thailand's speedboat ''captains'' is being questioned more frequently, with increasing accusations that they are poorly trained and do not take their job seriously enough.
A local authority, Marine Office 5, is supposed to be in charge of safety and training, however its Phuket chief, Phuripat Theerakulpisut, has been known to describe a speedboat injuring a swimmer with a propeller off a Phuket beach as ''natural.''
Khun Phuripat told a safety meeting of speedboat company representatives last week that he had planned to suspend the speedboat captain for three months after the collision between the jet-ski and the speedboat off Patong.
The man's Ship Licence had expired.
''I told the man he could not drive the boat for three months, and he burst into tears,'' Khun Phuripat said. ''So I reduced the penalty to one month.
''He was still sobbing, so I made it a week. Then his eyes were still very red from all the crying, so I told him to bring his Ship Licence up to date right away.''
As yesterday's crash did not take place in Phuket waters, officials from Krabi or Phang Nga are likely to be involved.
A number of Chinese have also drowned on snorkelling day-trips in the past 18 months, causing concern that not enough effort goes into protecting tourists who can't swim or can barely swim.
The deaths of the two tourists in Pattaya became the first case resolved at a new Tourism Court, designed to speed criminal and civil cases involving visitors.
A new Tourism Court opens on Phuket tomorrow, however it's unclear whether the case will be dealt with on Phuket if the collision occurred in waters that form part of the neighboring province of Phang Nga.
The British Embassy is supporting a planned marine safety summit for Phuket in October. Confirmation of the summit awaits formal approval by Phuket authorities.