Blastocrithidia is a genus of parasitic flagellate protist belonging to the family Trypanosomatidae. It is a monoxenous parasite of heteropteran insects, mainly inhabiting their hindgut and glands.

In addition to Blastocrithidia, one-host trypanosomatids from hemipteran and dipteran insects have been traditionally placed in genera Crithidia, Leptomonas, Herpetomonas, Rhynchoidomonas, and Wallaceina. Blastocrithidia is characterized by epimastigote morphological forms, whereas opisthomastigotes and endomastigotes are exclusive features of the genera Herpetomonas and Wallaceina, respectively. Blastocrithidia is also able to produce resistant cysts.

The etymology of the genus name Blastocrithidia derives from the two Ancient Greek words βλαστός (blastós), meaning "sprout, scion, child or descendant", and κριθίδιον (krithídion), meaning "small grain of barley".

The genus includes the following species.

Blastocrithidia uses in its nuclear genome an alternative genetic code characterized by all three canonical stop codons reassigned to sense codons.