Wiang Sa (Thai: เวียงสา, pronounced [wīa̯ŋ sǎː]) is a district (amphoe) in the central part of Nan province, northern Thailand.

Neighboring districts are, from the south clockwise: Na Noi, Ban Luang, Mueang Nan, Phu Phiang of Nan Province, Rong Kwang, Song of Phrae province.

The eastern part of the district is in the Luang Prabang Range mountain area of the Thai highlands.

[4] In 1939 it was again renamed with its historical name, only leaving out the word Mueang which was reserved for the capital districts of provinces.

[6] Wiang Sa is divided into 17 sub-districts (tambons), which are further subdivided into 128 administrative villages (mubans).