Thung Yai (Thai: ทุ่งใหญ่, pronounced [tʰûŋ jàj]) is the westernmost district (amphoe) of Nakhon Si Thammarat Province, southern Thailand.
Tambon Ku Rae was separated from Thung Song District and made the minor district (king amphoe) Ku Rae in 1906. When the office was moved to Tambon Tha Yang in 1909, the minor district was renamed following the central tambon name. However, when the government was about to upgrade the minor district, the name Tha Yang was already given to a district of Phetchaburi Province. Thus when the minor district was upgraded to a full district on 1 April 1961, it was named Thung Yai after one of the tambon of the district.