Portsdown is a 69.1-hectare (171-acre) biological Site of Special Scientific Interest on Portsdown Hill, on the northern outskirts of Portsmouth in Hampshire.

This is a linear south-facing escarpment with a rich chalk grassland flora. The diverse insect fauna includes all the chalk downland butterflies and a population of the largest British bush cricket, Tettigonia viridissima. On the lower slopes, raised beaches indicate former sea levels.

50°51′14″N 1°05′31″W / 50.854°N 1.092°W / 50.854; -1.092