Huaca Cao Viejo, Archaeological pyramid in Magdalena de Cao District, Peru.

Huaca Cao Viejo is a truncated adobe pyramid of the Moche culture in the Chicama Valley, built from seven superimposed structures over five centuries. Its final form reached 30 meters in height on a square base about 120 meters per side, with a long ramp on one side leading to the summit. The walls carry polychrome reliefs arranged in eight thematic sets, including human sacrifices, warriors, and Ai Apaec. In front lies a walled ceremonial plaza with a long platform and a small relief-decorated enclosure.

Construction began around 200 AD, and the huaca served as a Moche administrative and ceremonial centre controlling the Chicama valley. It was abandoned around 650, shortly before a strong El Niño whose torrential rains devastated the monument. Between the 8th and 14th centuries, Lambayeque people reused the walled ceremonial plaza as a cemetery.

The Lady of Cao, a woman of 1.45 m aged 20 to 25, was buried around 400 AD on an intermediate platform with 18 necklaces of gold, silver, lapis lazuli, quartz and turquoise, thirty gold and silver nose ornaments, and gilded copper crowns. Her body still shows tattoos of snakes and spiders, read as symbols of the earth's fertility or marks of a diviner. The mummy and its grave goods are displayed in the site museum.

The site lies 6 km from Magdalena de Cao and 60 km north of Trujillo, on the right bank of the Chicama River facing the Pacific. A long ramp on one side of the pyramid led to the summit, where a ceremonial courtyard, enclosure, main platform and altar stand. The Lady of Cao is displayed in a hall of the museum at the site entrance.

The Moche buried each older building under a new one, so the pyramid grew in seven superimposed phases over five centuries. Archaeologists led by Régulo Franco have studied the site since 1990, and in 2006 announced the tomb of the Lady of Cao, found on an intermediate platform.