Do my foreign documents need to be legalised for a Thai company?

Yes, for now. Documents signed abroad, such as a corporate shareholder's certificate or a power of attorney, are usually notarised and legalised through a Thai embassy or consulate. Thailand has joined the Apostille Convention, which enters into force for Thailand on 28 February 2027; from then an apostille should replace legalisation for most countries.

Certification and apostille

Source: Status table: Apostille Convention, Hague Conference on Private International Law (HCCH)