The first of this year's races, known as the Palio of Provenzano, in honour of the Madonna of Provenzano, took place on Sunday. For the first time, city authorities introduced a cap on the number of spectators allowed in the square, with metal detectors and additional security officers on hand. The event isn't without its controversy, and city authorities have passed regulations aimed at ensuring the animals' wellbeing after campaigners criticized the fact several horses had died in the race.
For the Siennese, the contest is a change to celebrate local pride and honour the city's long history.
Here's a look at this year's festival.
The day began with a costumed parade through the city centre, with participants carrying flags showing the symbol of their district or 'contrada'.
The parade or Corteo Storico includes around 600 people from musicians to flag-wavers, and leads spectators to the Piazza Campo where the race takes place.
Sebastiano Murtas, known as Grandine, stands next to his horse Querida de Marchesana, as it receives the traditional blessing before the race.
A local priest blesses the horse.
This year, just nine contrade were represented in the race, instead of the usual ten. Unfortunately for the Tartuca (tortoise) neighbourhood, slow and steady did not win the race, as the horse chosen to run refused to enter the track.
Each year, ten of the city's 17 neighbourhoods take part. The seven which didn't participate the previous year automatically get a spot, with the remaining three chosen by drawing lots.
A large crowd gathered at the Piazza Campo, the medieval square where the race is held.
Riders compete in the Palio, each wearing the traditional colours of their district.
The horses must take three laps around the square to complete the race.
Riders each have a long whip, which they are allowed to use not only to encourage their own horse, but also to distract or disturb other riders and their horses. There is huge rivalry between the districts, and the district that goes the longest time without a Palio victory gets the moniker 'nonna' (grandma). Currently, that title is held by the Aquila (eagle) contrada.
The name 'palio' comes from the prize given to the winner each year - a silk banner, usually painted by a local artist.
The winner is the first horse to cross the finish line - with or without its rider, who may well fall off due to the riding technique and fast speed.
After all the anticipation and build-up, the race is over very quickly, rarely lasting longer than a minute and a half.
This year's winner, Jonatan Bartoletti or Scompiglio, celebrates after winning the race for the Giraffa (Giraffe) district.
A victory in the Palio is a huge source of pride for the contrada. The Giraffa is one of only three districts to have ever won both races in a single year - will it be able to match that feat in the second Palio in August?
All photos, unless otherwise stated: Claudio Giovannini/AFP