Borsonella diegensis is a species of sea snail, a marine gastropod mollusk in the family Borsoniidae.

(Original description) The small shell is stout, solid, decollate, with a whitish substratum and strong olivaceous periostracum.

There is a faint spiral striation on the anal fasciole between the ridge and the keel.

The whole surface is covered with a microscopic, close, impressed, vermicular network of fine lines anastomosing in every direction.

The plication on the proximal part of the columella lags behind the aperture.