Seleucid dynasty (312–64 B.C.), dynasty in southwestern Asia, founded by Seleucus I (c.358–354 B.C.–218 B.C.). A general under Alexander the Great, Seleucus gained control of the eastern part of the Macedonian empire following Alexander's death (323 B.C.). The Seleucids came to rule the area between what is now Turkey and India. In the 200s B.C., the Parthians overran their eastern lands. In the 100s B.C., the Romans began to take over their western regions and brought the dynasty to a close with the occupation of Syria (64 B.C.). The Seleucids helped to spread Greek culture to western Asia.