Ton-That () and Tôn Nữ (for female) is a two-character Vietnamese compound surname, originating from the Nguyá» n dynasty. It is the surname for members of the imperial family that were not direct first-born descendants of the Emperor, and therefore considered collateral relatives of the Nguyá» n dynasty.
This surname was originally Tông Thất (), which is derived from a Sino-Vietnamese word, meaning "clan members" or "royal family members". The surname was changed to Tôn Thất () after Thiá»Âu Trá» (né: Nguyá» n Phúc Miên Tông) became the emperor due to naming taboo.