Grimstad (pronounced [ˈɡrɪ̀mstɑ] ^ⓘ) is a municipality in Agder county, Norway. It belongs to the geographical region of Sørlandet. The administrative center of the municipality is the town of Grimstad. Some of the villages in Grimstad include Eide, Espenes, Fevik, Fjære, Håbbestad, Hesnes, Homborsund, Jortveit, Kroken, Landvik, Nygrenda, Prestegårdskogen, Reddal, Roresand, Rønnes, Skiftenes, Tjore, Vik, and Østerhus.

The municipality is centered around the little maritime town of Grimstad which is surrounded by many small islands (Skjærgård). There is a harbor, a main street, a small market square, Grimstad Church, and a museum dedicated to the early life of Henrik Ibsen, who served as an apprentice to Grimstad's local pharmacist Reimann, from 1844 to 1847, before leaving Grimstad in 1850. Ibsen's intimate knowledge of the local people and surroundings can be seen in his poem Terje Vigen. The majority of the inhabitants live in and around the town, while the rest of the municipality is rural and heavily forested.

The 304-square-kilometre (117 sq mi) municipality is the 264th largest by area out of the 356 municipalities in Norway. Grimstad is the 50th most populous municipality in Norway with a population of 24,587. The municipality's population density is 90.3 inhabitants per square kilometre (234/sq mi) and its population has increased by 13.9% over the previous 10-year period.

The town of Grimstad was established as a municipality on 1 January 1838 (see formannskapsdistrikt law). On 1 January 1878, part of the neighboring municipality of Fjære (population: 948) was transferred to Grimstad. Again, on 1 January 1960, another part of Fjære (pop: 344) was transferred to Grimstad. On 1 January 1971, the rural municipalities of Fjære (pop: 6,189) and Landvik (pop: 2,781) were merged with the town of Grimstad (pop: 2,794) to form a significantly larger municipality of Grimstad with a total population of 11,764 at the time of the merger.

The municipality is named after the historic town of Grimstad (Old Norse: Grómstǫð). The first element is the name of after the old Grøm farm. The farm name is the combination of two items. First, the old local river name Gróa which comes from the word gróa which means "to grow" and the second part is heimr which means "home" or "abode". The last element of the name is stǫð which means "landing place" or "harbour". The town's name was originally spelled Grømstad, when Norway was part of the Kingdom of Denmark-Norway. At some point, the spelling of the name was misunderstood and became spelled as "Grimstad" during the registration of Norwegian cities and small places.

The coat of arms was granted in 1899 and was based upon a seal of the city dating back to 1847. The blazon is "Azure, a brig above three barrulets wavy Or". This means the arms have a blue field (background) and the charge is a two-masted brig, a type of sailing ship, on top of three wavy lines which symbolize the ocean. The charge has a tincture of Or which means it is commonly colored yellow, but if it is made out of metal, then gold is used. The arms usually have a mural crown above the shield. The design was chosen as a symbol for the importance of fishing and shipping.

The Church of Norway has four parishes (sokn) within the municipality of Grimstad. It is part of the Vest-Nedenes prosti (deanery) in the Diocese of Agder og Telemark.

Grimstad lies within the boundaries of the ancient parish of Fjære. It is reportedly first mentioned as a harbor in the 16th century. Eight years after he was deposed, Christian II of Denmark–Norway (1513–1523) attempted to recover his kingdoms. A tempest scattered his fleet off the Norwegian coast, and on 24 October 1531, they took refuge at Grimstad. On 1 July 1532, he surrendered to his rival, King Frederick I of Denmark, in exchange for a promise of safe conduct. King Frederick failed to honor his promise and imprisoned Christian until he died.