Do I need a return ticket when flying to Thailand on a one-way ticket?

Yes they will, unless you have a visa indication your ok to stay.

Came on one way. Never asked.

I’ve flown on a one way ticket to Thailand many times in the last 10 years, most recently two months ago, and they’ve never checked or ever even asked me anything. That’s not to say they won’t though, but it’s extremely unlikely. Your best bet is to buy an onward ticket to Vietnam for like $50.

I came without one time. Airline asked and I said I was leaving by land border.

Many people have suggested one of the onward ticket sites. Just be aware that every onward ticketing site only makes an on-hold reservation that shows as pending, not an actual paid ticket with an e-ticket number. This means there is always a chance that a knowledgeable person reviewing your proof of onward travel will know it isn't a valid ticket and is nothing more than a hold. You could actually do the same thing yourself by booking a flight on any airline website that allows a ticket hold for a few days or that allows a ticket hold to pay at the counter or at a store. It would generate the same reservation with a valid PNR booking code pending your payment. If you are using it at an airline check-in counter when they ask for an onward ticket, the check-in staff usually knows to look for an actual e-ticket number as PNR codes don't indicate it is an actual ticket. Onward ticket sites work well when applying for visas where consulate officers typically don't know the difference, but it is risky using it at an airport check-in counter when you have a one way ticket.

What Visa will you be entering with?

Just get a fake one. Works great. Five bucks.

SOMETIMES airline will ask for forward or return>if you dont have a visa

Use these guys

When you checkin and give them your passport, they will check to see if you have a return ticket in your reservation. At that time they will ask you for whatever documentation you have to allow you to stay in Thailand if you don’t have a return ticket.

Buy a cheap ticket out of the country, it may be wasted money but it may save you a lot of stress

If you have a Thai wife get a non-immigrant O spousal visa in USA then you should be fine.

It’s down to the airline to check for this usually.

If you have a non-immigrant visa then you won't need proof of onward travel out of Thailand.

If you have a tourist visa or are traveling without an visa at all, then your airline might require proof of your ticket out of Thailand before they'll allow you to board your flight.