From the US State Department page on visiting Thailand

ENTRY / EXIT REQUIREMENTS FOR U.S. CITIZENS: If you are a U.S. citizen tourist staying for fewer than 30 days, you do not require a visa, but your passport must be valid for at least six months beyond the date of your entry into Thailand. You may be asked to show an onward/return ticket.

Passports should only be considered valid for nine years and six months.