I'm flying to Bangkok from the US, and plan to stay in the region (Thailand and neighboring countries) for 3-4 months.

I'll probably stay in Thailand for less than a month, go to some neighboring countries and return to Thailand later.

Simple question, but is it OK if the return ticket is 3-4 months after the arrival date? (Even though the visa is just valid for 1 or 2 months.)

It would be OK if you had a Tourist Visa in your passport. If arriving without a visa, technically you need a return ticket within 30 days.

You need at least a single entry tourist visa to even be sure you can board the plane in the US with a return over 30 days from departure. The problem isn't entering Thailand as they will stamp you in for 30 days without a visa; the problem is being boarded in the first place. Depending on your exact dates, you might want to consider a double entry tourist visa. Also note that when traveling in the region, entries by land get you 15 days stay, by air 30 days without tourist visa, and 60 days with tourist visa. If you enter with a 60 day tourist visa and leave Thailand in 30, the 60 day permission to stay ends unless you obtain a reentry permit at a Thai immigration office in Thailand (if you do the same 60 day ending date applies). Also you can extend any tourist visa by 30 days within Thailand at an immigration office. Have fun.

If you want to attempt to board without any visa, you can purchase an AIR ticket leaving Thailand to a neighboring country leaving within 30 days, and with that you should be boarded in the US without a visa.

Edited November 11, 201114 yr by Jingthing