Prenuptial agreements: register it with the marriage or it is void

Thailand allows prenuptial agreements, but with one unforgiving formality: under Civil and Commercial Code s. 1466 the agreement is void unless it is entered in the marriage register at the time the marriage itself is registered — either recorded in the register or made in writing, signed by both spouses and at least two witnesses, annexed to the register with a note of the annex made at registration. There is no fixing it later: you cannot register a prenup after the wedding, and once married the terms cannot be changed or cancelled except with court permission (s. 1467). The district office's own service guide lists the prenup among documents to hand over when you register (DOPA allows 20–30 minutes for a registration, longer if a prenup is involved).

What it can do: govern the couple's property relations instead of the statutory default. Without one, everything acquired during the marriage, gifts and bequests expressly given as marital property, and the fruits of personal property are sin somros (s. 1474) — and in case of doubt an asset is presumed sin somros. A prenup registered under ss. 1465–1466 is also the only way spouses can manage marital property differently from the default joint-management rules (s. 1476/1). For a mixed-nationality couple the practical advice is to prepare a Thai text (with a translation you both actually understand) before the amphoe visit, because the registrar records the Thai version.