The two monoliths in red quartzite were raised at what European and Egyptian archaeologists said were their original sites in the funerary temple of the king, on the west bank of the Nile.
The temple is already famous for its existing 3,400-year-old Memnon colossi -- twin statues of Amenhotep III whose reign archaeologists say marked the political and cultural zenith of ancient Egyptian civilisation.
"The world until now knew two Memnon colossi, but from today it will know four colossi of Amenhotep III," said archaeologist Hourig Sourouzian, who heads the project to conserve the Amenhotep III temple.
The two restored additions have weathered severe damage for centuries, Sourouzian said.
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"The statues had lain in pieces for centuries in the fields, damaged by destructive forces of nature like earthquake, and later by irrigation water, salt, encroachment and vandalism," she said.
One of the "new" statues -- its body weighing 250 tonnes -- again depicts the pharaoh seated, hands resting on his knees.
It is 11.5 metres tall, with a base 1.5 metres high and 3.6 metres wide. Archaeologists said with its now missing double crown, the original statue would have reached a height of 13.5 metres and weighed 450 tonnes.
Beside his right leg stands nearly a complete figure of Amenhotep III's wife Tiye, wearing a large wig and a long tight-fitting dress.
A statue of queen mother Mutemwya, originally beside his left leg, is missing, archaeologists said.
The throne itself is decorated on each side with scenes from that era, showing the unification of Upper and Lower Egypt.
The second statue, of Amenhotep III standing, has been installed at the north gate of the temple.