I have heard that I need some form of proof from the US that I am single to get legal marriage in Thailand--Is this a US requirement, or Thai. I checked the Embassy website and can not find any mention of this requirement.

It is a Thai requirement and is fully stated on the US Embassy website under citizen services/notary. Below is download link for the document - fill out and make appointment at Embassy to notarize it. It must then be translated into Thai and registered at the Ministry of Foreign Affairs. Then you can take it and spouse to a District Office and register a marriage.

http://bangkok.usembassy.gov/root/pdfs/marriage_affidavit.pdf

I have a related question, can anyone tell me if there is any time limit required after divorce (outside Thailand) before one can legally marry in Thailand. On the Thai Embassy website (London) it states a time of not less than 310 days - but this seems to be targeting the female party as this requirement can be waived with a doctors certficate stating that the applicant is not pregnant - I am pretty sure, being male, that I could get such a certificate but does anybody really know if this 310 day period is supposed to apply to the male party as well?

That part only applies to woman:

The law states: "In case of the woman ......................"

When I registered married at the Amphur last 15 years, the Amphur didn't ask us for this proof. If thing changes now?

But Before we married I did ask my husband, an American, to show me his divorce paper. so he got the paper that was issued by the US court, NOT from the Embassy.

When I registered married at the Amphur last 15 years, the Amphur didn't ask us for this proof. If thing changes now?

The purpose of getting proof from the US Embassy is for you to claim under oath that you are currently single and not married. This is a current requirement.

But Before we married I did ask my husband, an American, to show me his divorce paper. so he got the paper that was issued by the US court, NOT from the Embassy.

If he was legally divorced in the US, then he has a divorce paperwork from the court. In the US, the court will issue a divorce paper for each divorce. Having a legal divorce paper from a court does not mean that a person is currently single or not married. He or she may currently be married with a divorce paper from another marriage.

When I registered married at the Amphur last 15 years, the Amphur didn't ask us for this proof. If thing changes now?

But Before we married I did ask my husband, an American, to show me his divorce paper. so he got the paper that was issued by the US court, NOT from the Embassy.

It is certainly a requirement now. Note also that once you have the letter/document from your embassy you need to get it translated and then endorsed at the Ministry of Foreign affairs in Bangkok. If you do not live in Bangkok you can use a local translatoin service and they usually have a contact in Bangkok to get this done via courrier and power of attorney.

And it was also a requirement 15 years ago.

There is no way for the US Embassy (or you) to prove a negative; IE you can not show that you are not married, with any sort of document. How would anyone know that? There's no sort of giant US database that keeps such information.

What you can do is swear to a legal affidavit at the Embassy (in which case a lie would be perjury). This will suffice.

EDIT TO ADD: Sorry. Didn't see that my post was redundant.

I am a US citizen and my wife is Thai. We were married in Panama, where we currently live. We will be returning to Thailand to live in the next few months.

Will Thailand recognize this marriage? If so, what documents, stamps and seals will we need? Can the Thai Consulate here certify our marriage certificate?

It might not be a bad idea asking them. Basically your Embassy in Thailand has to sign off on a translation of the original marriage certificate and then it has to be legalized by the Ministry of Foreign Affairs here and then recorded at a District Office here if you intend to use marriage as an extension of stay reason with immigration.