Demetrius was born in either 275 or 274 BC and was the only child of King
Antigonus II Gonatas by
Phila, the daughter of
Seleucus I. He had an elder half-brother named Halcyoneus, but he died in an unknown battle sometime before the death of Antigonus in 239 BC. He had already distinguished himself during his father's lifetime by defeating
Alexander II of Epirus at Derdia and so saving Macedonia His nine year old successor, the future Philip V, was deemed too young to rule by the Macedonian nobility and so Demetrius' half-cousin,
Antigonus III Doson, was made regent. The exact location of Demetrius' tomb remains unknown, but was likely in
Beroea or
Aegae. ==Marriage and family==