Bamiyan, about 140 kilometres (90 miles) northwest of Kabul, is on the ancient Silk Route between Europe and Central Asia.
A team led by Zameryalai Tarzi had been searching for a fabled lying statue that measures 300 metres when they made the discovery, information and culture ministry advisor Mohammad Zia Afshar told AFP.
"The team found an 18- to 19 metre-long sleeping Buddha statue and 89 other relics, among them three coins from Greek, Bactrian and Islamic eras," he said.
A book written by a Chinese pilgrim who visited Afghanistan centuries ago suggested there existed a 300-metre-long Buddha in a sleeping posture in Bamiyan, which was once a centre for Buddhism.
Archaeologists had renewed their search for it after the collapse of Taliban regime in late 2001, which bombed and destroyed two giant standing Buddhas that were believed to be around 1,600 years old.
"The team excavated areas southeast of the 35-metre-tall destroyed Buddha and discovered the neck and right sho