Macedonian king of Egypt (reigned 116–110, 109–107, and 88–81 BC) who, after ruling Cyprus and Egypt in various combinations with his brother, Ptolemy X Alexander I, and his mother, Cleopatra III, widow of Ptolemy VIII Euergetes II, gained sole rule of the country in 88 and sought to keep Egypt from excessive Roman influence while trying to develop trade with the East.

The unusual will of Euergetes II partitioned Egypt's possessions, leaving Cleopatra III as the effective ruler of Egypt and Cyprus. Although she preferred his younger brother, Ptolemy Alexander, popular sentiment forced the dowager queen to dismiss him and to associate Ptolemy Soter on the throne with herself. After compelling the king in 115 to divorce his strong-willed sister-queen, Cleopatra IV, his mother forced Ptolemy to marry his younger, more pliable sister, Cleopatra Selene. The next year, after his brother was sent to Cyprus as governor, Ptolemy Soter appeared with his mother as joint ruler of Egypt. The latent hostility between the son and his mother finally erupted in October 110, when Cleopatra expelled him from Egypt and recalled his brother from Cyprus. Soter II returned in early 109 but was evicted anew by his mother in March of the following year.

After a reconciliation in May 108 he fled a third time and established himself in Cyprus, from where in 107 he invaded northern Syria to assist one of the claimants to the Seleucid empire, while his mother, allying herself with the Jewish king in Palestine, actively aided another Seleucid pretender. During the protracted war his mother died (101) and Ptolemy X Alexander became the sole ruler of Egypt, while Soter II remained entrenched in Cyprus.

After Alexander's unpopularity drove him from Alexandria a second time and he perished at sea, Soter returned to resume sole rule over Egypt. Lacking a queen, he brought back his brother's widow, who was also his own daughter, Berenice III, and associated her on the throne with himself. Shortly before Soter's return in 88 a serious native rebellion erupted around Thebes in Upper Egypt. After three years of hard fighting Thebes capitulated and was sacked in retribution.

Ptolemy Soter refused to give aid to the Romans in the course of their war with Pontus, a Black Sea kingdom, and after the Roman sack of Athens in 88 the Egyptian rulers helped rebuild the city, for which commemorative statues of them were erected. Ptolemy IX died in 81, leaving his daughter and widow as his successor.