If you have a proper tourist visa in your passport (ie not the free 30 day stamp) you can enter Thailand on a one way ticket. You have buy the tourist visa at a Thai consulate before you come here.

You can get into the country on a 30 day stamp as well, but only if you have an air ticket out of the country within 30 days...doesn't have to be a ticket back home, an air ticket to any country other than Thailand will do.

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“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.