Several hypotheses involving turnover, jumping master genes or occasional recombination explain the prevalence of undifferentiated sex chromosomes in poikilothermic vertebrates. Recent research has uncovered conserved heteromorphic or even homomorphic sex chromosomes in several clades of non-avian and non-mammalian vertebrates. Sex determination in sturgeons (Acipenseridae) has been a long-standing basic biological question, linked also to economical demands by the caviar-producing aquaculture. Here, we report the discovery of a sex-specific sequence from sterlet (Acipenser ruthenus). Using chromosome-scale assemblies and pool-sequencing, we first identified a ca. 16 kb female-specific region. We developed a PCR-genotyping test, yielding female-specific products in six sturgeon species, spanning the entire phylogeny from the basal branches (A. sturio, A. oxyrinchus) to the tips (A. ruthenus, H. huso), in species stemming from an ancient tetraploidization. Surprisingly, similar results were obtained in two formally octoploid species (A. gueldenstaedtii, A. baerii). The conservation of a female-specific sequence for a long period, representing 180 My of sturgeon evolution, and across at least one additional polyploidization event, raises many interesting biological questions. We discuss a conserved undifferentiated sex chromosome system with a ZZ/ZW mode of sex determination and potential alternatives.