The aculeate wasp family Sierolomorphidae is a small and enigmatic group including 14 species in 2 extant genera distributed in the Northern Hemisphere. Nothing is known about their biology, although ectoparasitoidism on insects has been speculated. Only 1 fossil genus fromTuronian (Upper Cretaceous) amber was tentatively assigned to this family. The family Sierolomorphidae is recorded from the Early Cretaceous for the first time, representing the earliest occurrence of the family. Orisolemorpha dyscheres Álvarez-Parra and Engel gen. et sp. n. is described and figured from a male in upper Albian amber of San Just in Spain and is distinguished from all other genera in the family. The genus Loreisomorpha is here assigned to a new subfamily, Loreisomorphinae Álvarez-Parra and Engel subfam. n., based on its significant differences in wing venation relative to the remainder of the family. Taxonomic placement of the new genus is discussed, including a key for the fossil sierolomorphid genera. The ecology and biogeography of the family are reviewed. A checklist is provided of all living and fossil Sierolomorphidae.