Dichomeris diacnista is a moth in the Gelechiidae family. It was described by Meyrick in 1923. It is found in Guyana. The wingspan is about 20 mm. The forewings are greyish-violet strongly strigulated with rather dark fuscous, the costal edge suffused ferruginous brown. There is an obscure rather dark fuscous spot in the disc at one-fourth and similar spots representing stigmata, the plical and second discal centred blackish, the plical and first discal transversely placed and confluent. There is also an indistinct darker subterminal shade and dark fuscous marginal dots separated by brownish-ochreous around the apex and termen. The hindwings are blackish-grey, rather thinly scaled.