The distance between Venice and Siena is roughly 200 miles (321 km), moving south from the Veneto region towards Tuscany. Train travel is best (4-4.5 hours) here, with a connection in Florence.
Self-driving can be burdensome due to the strictly enforced ZTL (limited traffic zones) rules across both historic cities. That said, the area surrounding Siena is full of worthwhile stops that necessitate a car, and hiring a private transfer is an option that removes the frustration of driving and parking. Ultimately, your choice of transportation largely depends on how much time you plan to spend.
Duration: 4-4.5 hours
This is the most convenient way to get from Venice to Siena.
There are no direct trains from Venice to Siena. For the first leg of the train, you will have to take the the high-speed train from Venice to Florence's Firenze S. M. Novella station. For this you can choose to depart from either Venice proper (Venezia S. Lucia) or Venice mainland (Venezia Mestre). You can choose to book it on Trenitalia's comfortable Frecciarossa or Italo's luxurious train. For both of these options, you can book your seat in advance.
For the second leg of the trip, however, you will only have the option of taking a regionale from Florence to Siena on Trenitalia as Siena is not on the high-speed train network and is only served by local trains. The regionale is the slower, less luxurious option (all trains have restrooms and air conditioning, and some are equipped with electricity sockets). Moreover, you will not be able to book your seat so the earlier you arrive the better; when seats run out, passengers are forced to make the trip standing up.
Once you arrive to Siena the walk from the station into the walled city center is about 25 minutes. To avoid the strenuous uphill walk you can opt for one of the "escalators" that lead you all the way into the city, clearly marked from the train station (5 minutes).
By Rental Car or Private Transfer
Duration: 3-4 hours (more with stops and traffic)
To get from Venice to Siena by car, take the A13 until you come across the Siena Nord exit that leads you into the city. This is a tolled route so make sure you have a Telepass badge or a credit card ready for the route.
The road between the two cities is rich with destinations and landmarks. You will be passing through the Emilia Romagna region that is popular for its balsamic and parmesan-rich cuisine. Bologna is directly on your route but you can also choose to detour into the nearby Modena. Once nearer to Siena, the hill town of San Gimignano is a must-see.
Hiring a private transfer is highly recommended for this route. If you choose to drive yourself, it is best to rent the car in Piazzale Roma once you are leaving Venice, as you will not be able to use it in the pedestrian-only Venezia S. Lucia. You will also be required to park it outside Siena's city center or to return the rental for an additional drop-off fee considering that Siena's city center is a ZTL zone. However, you may be able to enter with a car if your hotel grants you permission. In all cases, Siena is a small city that is best explored on foot.