Pesky question here. If you have a Tourist Visa with single entry and fly in via international flight, stay a couple days in Bangkok, then want to fly to Phuket....does that second flight require a visa with multiple entry or would just the single entry visa work? Thanks in advance.
TLDR : Answer Summary
If you arrive in Thailand on a single entry tourist visa, you do not need a new visa to travel from Bangkok to Phuket or any other domestic destination within Thailand. Your visa is valid for all travel within the country for the duration of its validity.