Constantine II (874?–952) was king of Scotland from 900 to 942 or 943. He was the son of King Aedh, first cousin of the previous king Donald II, and first cousin once removed of his successor Malcolm I, to whom he left his kingdom upon abdicating and becoming a monk. Constantine's reign is the second longest reign in Scottish history.

Constantine succeeded Donald to the Scottish throne in 900. If a coronation took place, then there is no surviving record of it.

During his reign, Constantine had to fend off Viking (or Norse) raids from the north and west. The earliest of these involved driving the Vikings away from Scotland, and this reached a triumphant climax at the Battle of Scone in 904, after which the Vikings were forced to withdraw from Scotland. However, by then, the Vikings had laid waste to much of Scotland, and in particular the Dunkeld and Alba areas.

Constantine later struggled to win land from, or at least not lose land to, his neighbour to the south, the Saxon area of Northumbria, where the Vikings, led by Viking king Ragnall , had resettled themselves. Constantine was involved in two battles with Ragnall (referred to as the Battles of Corbridge ) in that area in 914 and 918, both of which resulted in Scottish victories and ultimately a cessation of hostilities there with the Norse.

When he was not involved in fighting Vikings, Constantine remodelled the Christian church of the day to be more Gaelic in nature this included a Synod at Scone in AD906, and he introduced the mormaer (earls) system to Scotland.

Constantine married at some point in his life, but virtually nothing is known of it. The date and place of the marriage are unrecorded, and his wife's name is likewise forgotten. It is known, however, that the marriage produced at least three children, two sons and a daughter.

Constantine's daughter, whose name is also no longer known, married Olaf III Guthfrithson, the Norse king of Dublin at the time, in 937 in order to establish more stable relationship with the Norse. At least three children later came from this marriage. If it was intended to contribute to holding back Northumbria, it did not. Constantine was defeated at the Battle of Brunanburh by King Athelstan of England in 937. One of Constantine's sons, Cellach, died in this battle.

In 942 or 943, Constantine abdicated in favour of Malcolm and entered a Culdee monastery in St Andrews, Fife, and eventually became Abbot there. He died peacefully in 952, and was probably buried at the monastery. Constantine's surviving son, Indulf, later became King of Scotland.