While preparing the site at the ancient necropolis of Saqqara, the team also came across the door of another tomb that remains sealed.
Mostafa Waziri, secretary-general of Egypt's Supreme Council of Antiquities, said the intact door suggests the contents inside are likely untouched, and experts plan to open the door in coming weeks.
The tomb is thought to date back to the Fifth Dynasty of the Old Kingdom, which ruled Egypt from about 2,500 BC to 2,350 BC, not long after the great pyramid of Giza was built.
Saqqara is thought to have served as the necropolis for Memphis, the capital of ancient Egypt for more than two millennia.
Over the past six months archaeologists working at the site have discovered a wealth of new artefacts there, including both mummies and wooden statues of animals and birds.
Depictions of cats were particularly common at the site, reflecting the ancient Egyptian worship of the cat-headed god Bastet.
The team also found painted wooden cobra and crocodile sarcophagi, a collection of gilded statues depicting animal features, as well as objects including amulets, canopic jars, writing tools and papyri baskets.
Humans were mummified to preserve their bodies for the afterlife. Animals treated in this manner, such as the unusual scarab mummies, are thought to have been used as religious offerings.
"The (mummified) scarab is something really unique. It is something really a bit rare," said Mr Waziri.
"A couple of days ago, when we discovered those coffins, they were sealed coffins with drawings of scarabs. I never heard about them before."
Scarabs also held religious significance in ancient Egyptian society, associated with the sun god Khepri.
The tombs in which these discoveries were made lie in a buried ridge that has only partially been uncovered and could offer many more similar discoveries, according to Mr Waziri.
Excavations in the area had halted in 2013 before resuming earlier this year.
Egypt has been whipping up publicity for new historical discoveries in recent years in an attempt to revitalise its tourism sector, which has still not recovered since the 2011 uprising that toppled president Hosni Mubarak.
Additional reporting by agencies.