Modes of sex determination in reptiles have undergone rapid turnover throughout their evolution. In some species, sex is determined genotypically by the presence of sex chromosomes (GSD). Alternatively the environment (usually temperature) may be the primary driving force over male and female development (TSD). The recent discovery of transitional modes of reptile sex determination suggests that there is not a strict dichotomy between GSD and TSD. In the Central Bearded Dragon (Pogona vitticeps), exposure to high temperatures during embryonic development can override genotypic sex determination and cause male-to-female sex reversal despite the presence of well-characterised ZW sex chromosomes. Here we provide the first evidence that temperature sex-reversal occurs in the wild and that sex-reversed individuals can successfully reproduce. Through laboratory manipulations we show that sex-reversed ZZ females produce clutches of offspring with extreme male-bias (97.5% male) at low incubation temperatures; whereas high incubation temperatures produce female-biased clutches via sex reversal (82% female), thus achieving a classic TSD response curve. When the environmental conditions for sex-reversal occur and persist, the transition between modes could be quickly reinforced as the proportion of ZZ individuals increases in the population and drive the W chromosome towards its eventual loss.