The ministry of antiquities said the chamber at the Dahshur royal necropolis, south of Cairo, contained a wooden box engraved with hieroglyphs.
Inside the box were four canopic jars filled with the organs of the deceased, likely a daughter of King Emnikamaw.
The ruler’s pyramid is about 600m (1,970ft) from the chamber.
Last month, archaeologists investigating the remains of the structure found a relief with 10 lines of hieroglyphs bearing Emnikamaw’s name.
They also uncovered the remnants of an anthropoid sarcophagus.
Dahshur is where King Sneferu of the 4th Dynasty built ancient Egypt’s first true smooth-sided pyramid, the 104m-high (341 ft) Red Pyramid, about 4,600 years ago.
He also constructed an earlier version, the 105m-high Bent Pyramid, whose slopes change angle from 54 degrees to 43 degrees about halfway up.
Sneferu was succeeded by his son Khufu, renowned for the Great Pyramid at Giza, which – at 138m high – was a wonder of the ancient world