Leioproctus castaneigaster, or Leioproctus (Leioproctus) castaneigaster, is a species of bee in the family Colletidae and subfamily Colletinae. It is endemic to Australia. It was described by entomologist Michael Batley in 2025.
Etymology The specific epithet castaneigaster (Latin: "chestnut belly") refers to metasomal colouration.
Description The holotype male body length is 7.8 mm, head width 2.5 mm; female body length 9.7 mm, head width 3.0 mm. Integumental colouration is mainly black with brown markings, the metasoma partly chestnut-brown, especially on females. The hair is mostly brown.
Distribution and habitat The species occurs in eastern New South Wales. The type locality is Cunnawarra National Park. It has also been recorded from Clarence and Dorrigo National Park.
Behaviour The adults are flying mellivores. Flowering plants visited by the bees include Coronidium elatum, Leucopogon affinis and Boronia microphylla.
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