@Maksym **********

In February I was denied check in to KUL from BCN with Qatar Airways for not having outbound ticket from KUL . I purchased a ticket KUL - SIN on the spot and they allowed me to checked in with no worries. Qatar said this was a requirement from Malaysian authorities.

Thai authorities formally require tourists to have a return or onward ticket from Thailand in order to enter the country by air. Although proof of this ticket is not usually requested at the immigration control of entry to Thailand, it is recommended that you comply with this requirement in any case.

Not as strict as in Malaysia, for example, where a return or onward ticket is a must. All airlines bound for Malaysia will request this requirement when checking in. No return or onward ticket you will be denied check in.