Hylaeus semicastaneus, or Hylaeus (Rhodohylaeus) semicastaneus, is a species of bee in the family Colletidae and subfamily Hylaeinae. It is endemic to Australia. It was described by British-American entomologist Theodore Dru Alison Cockerell in 1918.

Description Female body length is about 6 mm. Integumental colouration is mainly black with red markings, the abdomen chestnut red.

Distribution and habitat The species occurs in eastern Australia. The type locality is Brisbane in south-eastern Queensland.

Behaviour The adults are flying mellivores.

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