Unaizah or officially The Governorate of Unaizah (also spelled Onaizah, Onizah, or Unayzah; Arabic: محافظة عنيزة‎) is a Saudi Arabian city in the Al Qassim Province. It lies south of the province capital Buraydah and north of Riyadh, the capital of the Kingdom of Saudi Arabia. It is the second largest city in Al-Qassim Province with a population of 163,729 (2010 census).

Historically, Unaizah was an important stopping point for Muslim pilgrims coming from Mesopotamia (now Iraq) and Persia (now Iran) on their way to Makkah. Many scientists and historians believe that Unaizah was inhabited hundreds of years before the spread of Islam, citing its reference in numerous poems from some of the most important poets of pre-Islamic Arabia such as Imru Al-Qais.

Unaizah is in the south of Al-Qassim Province and at the heart of the historical region of Najd. It is located roughly 30 kilometers from Buraydah (the capital of the province) and more than 300 kilometers north of the Saudi capital, Riyadh. Unaizah lies in the northern-central region of the Najd and to the south of the Wadi Al Rumma (Rumma Valley), which is the longest valley on the Arabian peninsula. It is surrounded by sand dunes to its north and west, which are known locally as the Al-Ghamis Sands. The Al-Ghadha Woods are located to the north of the city.Next to Unaizah is the Al-Aushaziyah salt lake (or Sabkha), which is considered an official part of the city.