Girolamo Cassar (Maltese: Ġlormu Cassar, c. 1520 – c. 1592) was a Maltese architect and military engineer. He was the resident engineer of the Order of St. John, and was admitted into the Order in 1567. He was involved in the construction of Valletta, initially as an assistant to Francesco Laparelli, before taking over the project himself. He designed many public, religious and private buildings in the new capital city, including Saint John's Co-Cathedral, the Grandmaster's Palace and the auberges. He was the father of Vittorio Cassar, another architect and engineer.