The disaster was the latest in a series that have called into question Taiwan's safety record.
Media footage showed the bus, with flames shooting from the front, had rammed into an expressway barrier near Taipei.
The images showed thick plumes of smoke and burned-out wreckage at the roadside.
A police spokesman said the bus had caught fire before it crashed into the barrier but gave no reason.
"All the people on the bus died," Lin Kuan-cheng, spokesman for the National Fire Agency, told AFP.
"At this stage it is still not clear why no passengers escaped from the bus."
The bodies of the victims remained inside the bus as police and prosecutors examined the site, said an AFP photographer at the scene.
One image in Hong Kong's South China Morning Post newspaper showed two men trying to smash the windows with fire extinguishers as the doors of the vehicle remained shut.
The Liberty Times newspaper quoted an unnamed eyewitness as saying passengers were pounding the bus windows for help as the driver swerved sharply before the crash.
A firefighter at the scene said there were no survivors still calling for help when they arrived.
The tour group of 24 people - three children, 15 women and six men - was from China's northeastern city of Dalian, Taiwan's interior ministry said.
A Taiwanese driver and Taiwanese tour guide were also killed, the National Fire Agency confirmed.
The group were on their way to Taipei's main Taoyuan airport for a 4:30 pm flight back to Dalian after an eight-day tour of the island. The accident happened shortly before 1:00 pm.
String of accidents
Chinese tour groups have increasingly visited Taiwan in recent years after a boom in mainland tourism.
That was fostered by a rapprochement between the rivals under former Taiwanese President Ma Ying-jeou, who came to power in 2008 and left office in May.
There are fears the industry may be hit after Beijing-sceptic Tsai Ing-wen won the presidency in January, amid reports that tourist numbers have dropped.
Taiwan's Mainland Affairs Council, the official body handling cross-strait relations, said it had informed its counterpart in China of the crash.
Several recent fatal accidents in Taiwan have led to safety probes.
The collapse of a residential block during an earthquake in Tainan in February, which left 115 dead, led to an investigation which showed builders had cut corners.
In June 2015 coloured corn starch sprayed over crowds at a water park party near Taipei ignited due to the heat of stage lights, killing 15 and injuring more than 500 - many of them young people who sustained horrific burns. The organiser of the event was jailed for negligence.
In February 2015 a Transasia plane crashed into a river in Taipei, killing 43 on board. A recent report by investigators confirmed the pilot had shut down the wrong engine after the other one failed. The airline was instructed to overhaul safety procedures and training.
