Can a second wife be registered at a Thai district office (amphoe)?

No. The district office will not register a second marriage of a man who already has a registered marriage. The Thai Civil and Commercial Code rests on the principle of monogamy: s.1452 (amended 2024) states that a person may not marry while still having a spouse, and a marriage in breach of it is void (s.1495). A civil marriage exists only once registered (s.1457). As a result, most polygynous marriages in Thailand have only a religious certificate from the Provincial Islamic Committee. In the four southern provinces, the courts can recognise it in family disputes between Muslims, but for other civil matters a wife without civil registration is in a weak position (please check current practice). See Thailand.