I'm on a 60-day tourist visa; the stamp in my passport says it is valid until May 25.

I had originally planned on flying out of Thailand today, but my plans fell through and I'd rather not book a new flight.

Am I right in assuming that I can go to Thai immigration on the 25th and extend my visa for 30 days. I.e. The visa is still valid on the 25th and I wouldn't have had to go today correct?

Thanks!

Yes, you can go to immirgaiton today or tomorrow and apply for a 30 day extension for 1,900 baht.

Yu will need to bring a passport size photo and copy of passport and arrival card.