Do I need to book a return flight to enter Thailand on a tourist visa?

I had no problem flying one way from UK to Thailand on 30 day entry visa no questions asked but sometimes they do

Been 9 times in last 25 months ( from Uk. ) never been asked for return ticket

if you have a visa - no

The airlines when you check in for your flight if you do not have travel booked out of your destination will require proof that you have a right for long stay in that country. This can be a retirement visa, citizenship or right for long term stay. At your destination immigration can ask you for your outbound travel. If you do not have proof of outbound travel you can be denied entry. If you are denied entry it is the airlines responsibility to put you on the next available return flight. The airline is the one that has to pay for the ticket and airline can be fined or worst case have their landing rights taken away if they have to many infractions of not checking passengers properly when they checkin.

I came here last month from Arizona. Round trip ticket 35 days out for return. Delta Airlines asked for my visa. I told them visa exempt for 30 days and would get an extension. They put in a call to a supervisor who okayed it.

Perhaps. Book a cheap air asia flight to Cambodia and cancel it on arrival

Flying Korean Airlines three months ago...one way tic and never asked for a return flight. Depends on the airline, and maybe other factors too.

Times have changed, I came here in 86 on a one-way ticket with 100 quid, ....still here 🕶

Yes and why?

Maybe on check in if u dont have a visa> If going for the visa exempt use

Immigration stopped me to check my return flight details had to prove that had an e-ticket for return journey

If going visa exempt you may have problems with the airline and/or immigration. I have been asked for this several times in the past year.

So be prepared to possibly get sent home….

If you don’t have a long term visa then you will have problems at your point of departure

Yes

Depends on what visa you'll have when you fly. If you have a long term visa non-immigrant visa then onward travel from Thailand is not required.

If you're flying with a tourist visa or no visa, your airline might require you to show proof you're leaving Thailand in 60 or 30 days or they won't let you board your flight. Contact them to find out their policy.