Liverworts comprise one of the six primary lineages of land plants, with the predicted origin of the extant diversity of liverworts dating to the Silurian. The ancestral condition of liverworts is dioicy (unisexuality). In dioicious liverworts sex is determined chromosomally, with the female possessing a U chromosome and males having a V chromosome, and the ancestral dioicious liverwort likely harboured dimorphic sex chromosomes. Monoicy (bisexuality) has evolved multiple independent times within liverworts, prompting the question of the fate of the highly dimorphic sex chromsomes during the evolution of monoicy from ancestral dioicy. Here we explore the genomic consequences of the evolution of monoicy, focusing on the monoicous species Ricciocarpos natans. We propose that the evolution of monoicy in R. natans occurred via a fusion of the U and V chromosomes in an aneuploid spore and retention of this fusion chromosome harbouring essential gametologs and V chromosome genes, with subsequent breakage-bridge-fusion cycles distributing remaining essential U chromosome genes to autosomal locations. A survey of other monoicous liverworts suggests similar genomic rearrangements may have occurred repeatedly in the evolution of monoicy from dioicous ancestors.