Florence International Airport 86km: You can either take the bus or the train. However they don’t stop at the same finishing point. The bus will drop you off in the heart of the historic city centre of Siena whereas Siena train station is located further away from the city centre and you might need to take a city bus (5 min travel time approx).
– Autostradale bus express, coming from Pisa, stops in Florence and reaches Siena in less than 2 hours. See: www.airportbusexpress.it.
– SITA bus, www.sienamobilita.it. The bus station is just around the corner from the train station in Florence. SITA buses to Siena take just under an hour, run frequently and takes you to Siena historic city center. Make sure you take the rapide (express) and not the ordinarie, which stops in Poggibonsi and Colle Val d’Elsa. Ticket costs €12. See the route itinerary and price for more information (note information is only in Italian). To reach the Santa Maria Novella train station in Florence from the airport, just take the city line Vola in bus from the airport. The service connects the city centre in just 20 minutes and the fare is €6 one way.
Trains from Florence to Siena leave Santa Maria Novella station roughly once an hour on most days, and a direct train takes about 1.5 hours. The ticket is in the €6-10 range (if you’re on the direct train). See www.fsitaliane.it for more information.
Pisa International Airport 130km: You can take two different buses outside the airport going directly to Siena (under 2hr approximate travel time), for more information, see: http://www.pisa-airport.com/aeroporto_di_pisa_196.html.
– Train S.p.a, see: www.trainspa.it. Leaving directly from the airport to Piazza Gramsci in Siena. It takes you around two hours travel time and costs €14 one way.
– Autostradale, see: www.airportbusexpress.it. It follows similar itinerary than Train S.p.a but with a change in Florence. One way ticket costs €15. All informations, timetable and route with Siena Mobilita.
Bologna International Airport 167km: There are bus and train connections available to Siena. Shuttle bus from Bologna Airport to Bologna Central Station (every 11 mins, €6 one-way). For more information see: http://aerobus.bo.it/en. From here you will take the train to Siena with a change in Florence. Train to Siena (2hr 30, from €24 one-way). For more information see: https://www.lefrecce.it.
Bus from Bologna Airport to Siena Train Station. (4 times daily, from €18, Duration 2h40 – 3h40). For more information see: http://www.sena.it/Autobus/Bologna/Siena/125-234/49-1.html.