Porirua Harbour is a natural inlet in the south-western coast of the North Island of New Zealand. The city of Porirua, one of the four cities in the Wellington conurbation, surrounds it. The city centre is to the south of the harbour.
The harbour has an entrance only a few hundred metres in width, close to the suburb of Plimmerton. It opens up into two arms, Porirua Inlet to the south and Pauatahanui Inlet to the north-east. Each arm is around three kilometres in length.
The wetland at Pauatahanui, where Pauatahanui Stream enters the Pauatahanui Inlet, is the largest remaining estuarine wetland in the lower North Island, and the Pauatahanui Wildlife Reserve has been established to protect the inlet's environment and to restore damaged areas.
Part of the Porirua Inlet was reclaimed for a causeway for the North Island Main Trunk Railway. The construction of this causeway created three shallow lagoons from the harbour. When State Highway 1 was re-aligned in the 1970s alongside the railway, these lagoons were partially filled in. Aotea Lagoon was developed into a recreational area, albeit very polluted.