The coach was carrying some 60 Eastern European tourists and their guide when it overturned near the small Tuscan town of Monteriggioni at around 9:30 am, according to reports.
One woman was found dead at the scene and another 31 people are reported injured. Fifteen are said to be in a serious, though not life-threatening, condition.
Some passengers remained trapped in the vehicle after it tipped onto its right side, firefighters said, as they and other emergency services worked at the scene.
#22maggio 11:00, intervento dei #vigilidelfuoco in corso dalle 9:30 a Monteriggioni (SI) per un pullman uscito fuori strada sul Raccordo Autostradale 3, al km 7 in dir. Sud. Squadre al lavoro per soccorrere alcuni passeggeri incastrati nel mezzo pic.twitter.com/Lw5syOFc8U
— Vigili del Fuoco (@emergenzavvf) May 22, 2019
It's yet not clear what caused the crash. According to Repubblica, the coach is believed to have come off the road and crashed through a guard rail into the woods lining the motorway before it overturned.
The driver tested negative for alcohol, but “we're not ruling distraction as one of the causes”, said regional highway police chief Paolo Maria Pomponio.
“It could have been worse, the vegetation cushioned the vehicle's fall,” he added.
A stretch of the motorway heading south towards Siena has been partially closed while authorities investigate, causing traffic delays for several kilometres.
The group had set off from Montecatini earlier this morning and were headed for Siena as part of a tour of Italy's art cities.