Getting to Portugal from Spain

Train is 9 to 11 ish hours with 2 to 4 changes. Driving is 5½ hours. Bus is 8 hours. Check out Rome to Rio for specifics.

Driving also presents the high-cost of trans-border rental return fees.

Unfortunately, unless a journey through less-traveled parts of Spain is on the table, flying really is the best option for this.

Flixbus has two direct (no transfer) buses per day: https://shop.flixbus.com/. The first one departs Madrid’s South station at 10:00 arriving at 17:40 and another departing at 23:30 arriving the next morning at 6:40. I doubt if the seats recline in a horizontal position if taking the overnighter.

Flixbus - Looks like the seats recline, and there's free wi-fi. And have bathrooms on board. Doesn't look too bad to me if you need to get between the two destinations. Probably more legroom than most airline flights. And for an extra $16, you can travel "neighbor free" (book the seat next to you).

https://www.flixbus.com/service/onboard-services#:~:text=Buses%20are%20equipped%20with%20comfortable,comfortable%20even%20on%20longer%20trips.

Clarification – I doubt if Flexbus seats recline into a horizontal position.

It's pretty easy.

Step 1: Take the train from Madrid to Vigo. Several direct high speed trains daily that take you to Vigo in a bit over 4 hours.

Step 2: Travel from Vigo to Porto, either by train or coach, takes a bit over two hours for the last bit.

If you don't mind a bit of a detour, a stop in Santiago de Compostela is not a bad idea.

Unfortunately, Vigo seems quite a dull city. Not a bad place, just not somewhere you'd want to spend time. Try not to end up with too much time to kill there. There are lots of very good destinations in that part of Spain (Galicia), starting with the aforementioned Santiago de Compostela; Vigo's just not one of them.

Although not a really large place, I believe Vigo still has two train stations. A rail connection there might require a station change as well.

I wouldn't trust any of the details provided by Rome2Rio. The travel times, frequencies and fares it displays can be wildly off-base. It sometimes gives accurate bus-company names.

Correct, there are two stations in Vigo. But they are pretty close to each other, so it's not a big problem if your trains use different stations.

And while I agree that Vigo is not the most attractive place in Spain, it's not a bad place to have lunch between your trains. And if you have a few hours to spare, the old fortress is not a bad place to visit.

Thank you for all of you suggestions!