Mangua caswell is a species of Physoglenidae spider endemic to New Zealand.
This species was described in 1990 by Ray Forster from male and female specimens. The holotype is stored in Te Papa Museum under registration number AS.000108.
The male is recorded at 3.49mm in length whereas the female is 3.61mm.
This species is only known from Fiordland, New Zealand.
Under the New Zealand Threat Classification System, this species is listed as "Data Deficient" with the qualifiers of "Data Poor: Size" and "Data Poor: Trend".