Mosterøy is a former municipality in Rogaland county, Norway. The administrative centre was the village of Askje where the Askje Church is located. The 23-square-kilometre (8.9 sq mi) municipality existed from 1884 until 1965. The island municipality included the islands of Mosterøy, Klosterøy, Fjøløy, Sokn, Bru, and the western half of Åmøy. Today, Mosterøy is part of the municipality of Stavanger.

The Utstein Abbey, the best-preserved medieval monastery in Norway, is located on the island of Klosterøy. It was one of the most notable historic sites in the municipality.

The municipality of Mosterøy was established on 1 July 1884 when it was split off from the large island municipality of Rennesøy. Initially, the new municipality had a population of 1,309. On 1 January 1923, the westernmost group of outlying islands of Kvitsøy were separated from Mosterøy to form a separate municipality Kvitsøy. The split left Mosterøy with 745 inhabitants. On 1 January 1965, many municipal mergers took place in Norway due to the recommendations of the Schei Committee. On that date, Mosterøy was merged back into the municipality of Rennesøy. Prior to the merger, Mosterøy had a population of 817.

The municipality (originally the parish) is named after the island of Mosterøy (Old Norse: Mostr or Monstr) since the first Utstein Church was built there. The meaning of the first element of the name is uncertain. One possibility is that it is a shortened version of the Latin word monasterium which means "monastery", since the Utstein Abbey is located on the northwest end of the island. Another possibility is that it comes from the word mǫn which means "mane". A third possibility is that it comes from the word mœnir which means "ridge", likely referring to the 154-metre (505 ft) tall Mastravarden, the highest point on the island. The last element of the name is øy which means "island". Historically, the name of the municipality was spelled Mosterø. On 3 November 1917, a royal resolution changed the spelling of the name of the municipality to Mosterøy. The letter y was added to the end of the word to "Norwegianize" the name (ø is the Danish word for "island" and øy is the Norwegian word).

While it existed, this municipality was responsible for primary education (through 10th grade), outpatient health services, senior citizen services, unemployment, social services, zoning, economic development, and municipal roads. During its existence, this municipality was governed by a municipal council of directly elected representatives. The mayor was indirectly elected by a vote of the municipal council.

The mayors (Norwegian: ordfører) of Mosterøy (incomplete list):

• 1925-1931: Kristian Edland (V)
• 1931-1941: Lars Ramndal (V)
• 1945-1945: Lars Ramndal (V)

The municipal council (Herredsstyre) of Mosterøy was made up of 13 representatives that were elected to four year terms. The party breakdown of the final municipal council was as follows:

• List of former municipalities of Norway