Can I extend my single entry tourist visa upon arrival in Thailand?

Кіяк Бснцги yep, it's depending on the immigration office; Chiang Mai, Bangkok & Phuket will let you apply for a new extension when you have 45 days or less, most other offices will do it when you have 30 or less, and a few (Jomtien) will do it only when you have 15 days or less left.