Te Whanganui-A-Hei (Cathedral Cove) Marine Reserve is in the southern part of Mercury Bay on the Coromandel Peninsula in New Zealand covering an area of 840 hectares (2,100 acres). On the coast of the mainland, it stretches from Cook Bluff in the north-west to the northern end of Hahei Beach in the south-east. Its offshore extremes run from Motukorure Island through Waikaranga Island to Okorotere Island and the northern end of Mahurangi Island (Goat Island).

Part of the marine reserve lies off the Cathedral Cove Recreation Reserve, which runs from the northern end of Hahei Beach in the south-east to beyond Cathedral Cove in the north-west. With attractions such as a natural rock archway and neighbouring beaches at Cathedral Cove, the area is very popular with tourists, and receives around 150,000 visitors per year.

The walkway to the cove was closed due to damage from Cyclone Gabrielle in February 2023. In January 2024 the Department of Conservation said they were "not confident" the track would reopen due to significant damage to the stability of the ground in the area. An alternative viewing point has been established, and the cove can be viewed from boats, but access to the beach remains restricted.