Bus Options from Rome to Siena?

Check
www.Rome2rio.com for bus options

Or consider the train? From Termini change in Florence or Chiusi

Thanks, Christine! I've looked at the Rome2Rio site - looks like there are only tickets left for the Flixbus at 14:30pm. We were trying to avoid the train as it would be longer and we're thinking our kids (9, 12, and 16) would not appreciate that after a long flight.

I looked at Rome2rio and the bus and train are about the same amount of time. To me, a train is so much more comfortable as you can get up and walk around a bit.

I happen to have Rick's guide to Italy right in front of me. He mentions 13 buses (Flixbus) per day from Siena to Rome, though I suppose there might be fewer on Sundays. It seems pretty unlikely that all but one of the buses would be sold out unless you're dealing with something like a major holiday weekend--and even so, I wouldn't expect people to buy bus tickets terribly far in advance. What's your travel date?

Incidentally, it appears that some of the buses from Rome terminate at Piazza Gramsci in Siena, while others just go to the (less convenient) train station. You might want to factor that into your decision.

What time does your plane arrive at FCO?

Thanks for the information! We will arrive in Rome on 11JUN, early in the morning. I did see that Itabus hasn't put out their schedule yet for June - wonder if the same is true for Flixbus and they will have more options available once the calendar rolls over to March.

We've thought about the train, but it seems like overkill - going all the way to Florence, then back to Siena. I think worst case, we will find a place to stash our bags, then walk around the center of Rome for awhile before catching a later bus direct to Siena. Thanks, everyone!

Flixbus probably just doesn't have the full June schedule loaded yet. Take a look at the schedule for a Saturday in early April. (No sense confusing things by picking a date around Easter.) I'd think June would have at least as many buses as April.

You need not overshoot Siena by train and double back from Florence. There are regional routings that take a more direct route, albeit one that involves at least one transfer and slow(ish) trains. But the regional trains are considerably cheaper (Freccia trains are expensive, plus you'd be paying the extra mileage for your little jaunt up to Florence), and you don't need to commit to a specific departure time in order to get a ticket at a good price.

I'm thinking I'd probably choose the bus in this case, too. It's nice to be able to move around on a train, but this isn't a horribly long trip, and there's something to be said for not having to wrestle luggage up onto a train.

3 companies to choose from (in order of abundance of options):
https://www.flixbus.com/
https://www.itabus.it/
https://www.marinobus.it/en/

There are also consolidators (Getbybus, Busbud, Checkmybus, etc.) that sell tickets in the above companies. Check those too for best deals.