The Ptolemaic dynasty (; Ancient Greek: Πτολεμαῖοι, Ptolemaioi), sometimes also known as the Lagids () or Lagidae (; Λαγίδαι, Lagidai, after Lagus, Ptolemy I's father), was a Macedonian Greek royal family, which ruled the Ptolemaic Kingdom in Egypt during the Hellenistic period. Their rule lasted for 275 years, from 305 to 30 BC. They were the last dynasty of ancient Egypt.
Ptolemy, one of the seven somatophylakes (bodyguards) of Macedon who served as Alexander the Great's generals and deputies, was appointed satrap of Egypt after Alexander's death in 323 BC. In 305 BC, he declared himself Ptolemy I, later known as Sōter "Saviour". The Egyptians soon accepted the Ptolemies as the successors to the pharaohs of independent Egypt. Ptolemy's family ruled Egypt until the Roman conquest of 30 BC.
Like the earlier dynasties of ancient Egypt, the Ptolemaic dynasty practiced inbreeding including sibling marriage, but this did not start in earnest until nearly a century into the dynasty's history. All the male rulers of the dynasty took the name Ptolemy, while queens regnant were all called Cleopatra, Arsinoe or Berenice. The most famous member of the line was the last queen, Cleopatra VII, known for her role in the Roman political battles between Julius Caesar and Pompey, and later between Octavian and Mark Antony. Her apparent suicide at the conquest by Rome marked the end of Ptolemaic rule in Egypt.
Dates in brackets represent the regnal dates of the Ptolemaic pharaohs. They frequently ruled jointly with their wives, who were often also their sisters. Several queens exercised regal authority. Of these, one of the last and most famous was Cleopatra ("Cleopatra VII Philopator", 51–30 BC), with her two brothers and her son serving as successive nominal co-rulers. Several systems exist for numbering the later rulers; the one used here is the one most widely employed by modern scholars.
In continuation of the tradition established by previous Egyptian dynasties, the Ptolemies engaged in inbreeding including sibling marriage, with many of the pharaohs being married to their siblings and often co-ruling with them. Ptolemy I and other early rulers of the dynasty were not married to their relatives, the childless marriage of siblings Ptolemy II and Arsinoe II being an exception. The first child-producing incestuous marriage in the Ptolemaic dynasty was that of Ptolemy IV and Arsinoe III, who were succeeded as co-pharaohs by their son Ptolemy V, born 210 BC. The most famous Ptolemaic pharaoh, Cleopatra VII, was at different times married to and reigning with two of her brothers (Ptolemy XIII until 47 BC and then Ptolemy XIV until 44 BC), and their parents were likely siblings or possibly cousins as well.
Contemporaries describe a number of the Ptolemaic dynasty members as extremely obese, whilst sculptures and coins reveal prominent eyes and swollen necks. Familial Graves' disease could explain the swollen necks and eye prominence (exophthalmos), although this is unlikely to occur in the presence of morbid obesity. This is all likely due to inbreeding within the Ptolemaic dynasty. In view of the familial nature of these findings, members of this dynasty likely suffered from a multi-organ fibrotic condition such as Erdheim–Chester disease or a familial multifocal fibrosclerosis where thyroiditis, obesity and ocular proptosis may have all occurred concurrently.