was a feudal domain under the Tokugawa shogunate of Edo period Japan, in what is now western Ehime Prefecture on the island of Shikoku. It was centered around Niiya jin'ya in what is now part of the city of à Âzu, Ehime, and was ruled throughout its history by a cadet branch of tozama daimyà  Katà  clan. Niiya Domain was dissolved in the abolition of the han system in 1871 and is now part of Ehime Prefecture.
In 1617, à Âzu Domain was assigned to Katà  Sadayasu from Yonago Domain, with a kokudaka to 66,000 koku. Sadayasu died suddenly in 1623 without formally having appointed a successor, which would normally be cause for attainder; however, his eldest son Katà  Yasuaki managed to secure an audience with Shogun Tokugawa Hidetada and was accepted as heir. However, the shogunate also "informally" recognized his younger brother, Katà  Naoyasu, as daimyà  of his own 10,000 koku estate and to establish Niiya Domain and a cadet branch of the Katà  clan. This move was made without the prior knowledge of Katà  Yasuaki and was thus vehemently opposed by à Âzu Domain, and relations between the two branches of the clan was hostile for the next two centuries. Niiya Domain was left in the ambitious position of both being a subsidiary domain of à Âzu and a direct domain under the shogunate. The jin'ya was completed in 1642. The history of the domain was relatively uneventful. The noted Confucian scholar Nakae Tà Âju spent part of his early career in Niiya. In the latter half of the Edo period, the domain's finances were in serious trouble due to flooding and fires caused by the flooding of the Hiji River. During the Boshin War, the domain fielded a company of commoner riflemen in the pro-imperial army, but otherwise played no role. With the 1871 abolition of the han system à Âzu Domain became "à Âzu Prefecture" and merged with "Uwajima Prefecture" before becoming part of Ehime Prefecture. The Kato clan was awarded the title of viscount user the kazoku peerage system in 1884.
A building of former jin'ya of the domain survives, and is designated a Ehime Prefectural Tangible Cultural Property. It is located within the grounds of the Ozu Municipal Niiya Elementary School.
As with most domains in the han system, Niiya Domain consisted of several discontinuous territories calculated to provide the assigned kokudaka, based on periodic cadastral surveys and projected agricultural yields.