Pieter Isaacsz, was a Danish-born Dutch Golden Age painter.

According to van Mander his father was from Haarlem and he learned to paint in Amsterdam for a year and a half by Cornelis Ketel, and later by Hans von Aachen. Van Mander claimed he still lived in Amsterdam and went on to describe several portraits by him which he particularly admired, including a half-length portrait of Sara Schurmans playing a citar. His most popular piece was an oil-on-copper painting with a procession of angry women in Rome on hearing that the Senate had decided in favor of polygamy for men. According to Houbraken (who mistakenly called him Pieter Fransz) he later became the teacher of Adriaen van Nieulandt the younger.