Edaphic specialisation is one of the main drivers of plant diversification and has multifaceted effects on population dynamics. Carex angustisquama is a sedge plant growing only on heavily acidified soil in solfatara fields, where only extremophytes can survive. Because of the lack of closely related species in similar habitats and its disjunct distribution, the species offers ideal settings to investigate the effects of adaptation to solfatara fields and historical biogeography on genetic consequences of plant edaphic specialisation to solfatara fields. Here, genome-wide single-nucleotide polymorphisms were used to reveal the phylogenetic origin of C. angustisquama, and 16 expressed sequence tag–simple sequence repeat markers were employed to infer population demography of C. angustisquama. Molecular phylogenetic analysis strongly indicated that C. angustisquama formed a monophyletic clade with C. doenitzii, a species growing on non-acidified soil in sympatric sub-alpine zone. The result of population genetic analysis showed that C. angustisquama possesses much lower genetic diversity than the sister species, and notably, all 16 loci were completely homozygous in most individuals of C. angustisquama. Approximate Bayesian computation analysis supported the model that assumed hierarchical declines of population size through its evolutionary sequence. We propose that the edaphic specialist in solfatara fields has newly attained the adaptation to solfatara fields in the process of speciation. Furthermore, we found evidence of a drastic reduction in genetic diversity in C. angustisquama, suggesting that the repeated founder effects associated with edaphic specialisation and subsequent population demography bring the loss of genetic diversity to the extremophyte in solfatara fields.