I will be flying to Thailand from Jakarta tomorrow via singapore. Will I need to show an outbound ticket from thailand before boarding Thai-airways in singapore. ?

I know I'm fine getting on the plane in Indonesia , it's just the Thai airways in singapore i'm not sure about.

thanks for any advice.

If you have a visa, then no problem.

If you have no visa there could be an issue as an onward flight is a requirement of immigration for a visa-exempt entry, this is rarely checked by immigration, they rely on the airlines.

I have flown into Thailand many times per year for the last 10 years and I have never been asked by Immigration to show an onward ticket. Once United Airlines in L.A. wouldn't let me board a flight to Thailand without first purchasing a refundable onward ticket but of course when I got to Thailand Immigration did not ask to see it. I have flown into Thailand probably 70 - 80 times in the last 10 years and that was the only time anyone, airline or Immigration asked about an onward ticket. With typical United Airlines customer service it took them over a month to refund my money for the useless onward ticket that for some reason that one time they required me to purchase.

If Thai Airways allowed you to buy a one way ticket, they should also allow you to board the plane in Singapore. Just write a random outbound flight # on your immigration departure card and you should be fine. In Thai customs they normally don´t ask to see the outbound ticket. If any problems at check-in in Singapore, you can always just buy a refundable outbound ticket in Singapore.