Siphlonurus lacustris is a species of mayfly belonging to the family Siphlonuridae.

The adults of these aquatic insects reach 10–15 millimetres (0.39–0.59 in) of length and can mostly be encountered from early May through August, mainly in oxygenated waterways and in fresh water of lakes (hence the Latin name lacustris, meaning "lacustrine").

On the head they have large compound eyes and three ocelli.

The wings are held upright, they are triangular and translucent, with well drawn dark veins.

The female lay the eggs on the water surface and leave them sink to the bottom.