Mastalus I () (died 953) was the ruler of Amalfi from 900 to his death. He was the son of Manso I, who held the title of Prefect. In 900, he was elevated to co-rulership with his father. In 914, he became sole ruler, taking the title Patricius. During his lengthy rule, he co-ruled at times with his two sons, Leo and John, and his grandson, Mastalus. Upon his death, he was succeeded by his grandson, Mastalus II, who was raised to the status of dux (duke).
According to the Chronicon Salernitanum, in 946, Mastalus came to the rescue of Gisulf I of Salerno, who was assaulted by an alliance of Landulf II of Benevento and John III of Naples. Mastalus ambushed Landulf's forces in the pass at La Cava.