king of Norway (1103–22) whose reign with his brother Sigurd I Jerusalemfarer (Sigurd I Magnusson) was the longest joint rule in the history of Norway.

An illegitimate son of Magnus III Barefoot, Eystein succeeded to the throne in 1103 with his younger brothers Sigurd I and Olaf (IV); the latter, a child, died in 1115, but Sigurd outlived Eystein. While Sigurd was off on crusades in Moorish Spain and the Holy Land in 1107–11, Eystein served Norway with great ability, gaining territory from Sweden, building churches, and fostering internal progress.