Steeple Point to Marsland Mouth is a coastal Site of Special Scientific Interest (SSSI) in Cornwall, England, UK, noted for its biological characteristics.

It starts at Steeple Point in the south, following the shores of the Celtic Sea in the Atlantic Ocean to Marsland Mouth in the north, which is on the Cornwall-Devon border.

The SSSI also covers the Cornish side of Marsland Valley nature reserve, jointly owned by the Devon and Cornwall Wildlife Trusts.

[4] Amongst the cliff edge grassland buckshorn plantain (Plantago coronopus), carline thistle (Carlina vulgaris), red fescue (Festuca rubra), thrift (Armeria maritima) and wild thyme (Thymus praecox) can be found.

[4] The coastline of the SSSI supported one of the last known British colonies of breeding large blue butterfly (Maculinea arion), which was declared nationally extinct in 1979.