The Eritrean government says Foreign Minister Ali Said Abdella has died of a heart attack.
Minister of Information Ali Abdu called his death on Sunday a great loss for the country.
Ali Said Abdella apparently died in his sleep early Sunday morning.
Flags flew at half staff Monday as Eritrea began three days of national mourning. The foreign minister is to be buried on Tuesday.
During the 1970s and 80s, Mr. Ali was a commander with the rebel Eritrean People's Liberation Front, which later became the People's Front for Democracy and Justice, now the ruling party in Eritrea.
He became foreign minister in 2000 after serving in the provisional government as secretary for internal affairs.
Eritrea won its 30-year war of independence from Ethiopia in 1992.