In the sex-role reversed pipefish Syngnathus typhle females compete for access to males and males are choosy. Females display a temporary ornament, a striped pattern. We show here for the first time in a sex role-reversed species that ornament display predicts how much time a female will devote to competitive behaviours, that males prefer ornamented females over nonornamented ones, and that the ornament is attractive even when female behaviour is held constant. This was demonstrated in an experiment with a male choosing between two females, first with the females separated and then with interactions allowed between the females as well as with the male. Females displaying the ornament for a longer time enjoyed a higher mating success then those displaying the ornament more briefly. Ornament display in the absence of intrasexual competition also predicted the amount of time that females subsequently spent competing. Thus, females initially displaying an attractive trait were also subsequently competing for longer. Furthermore, we manipulated the ornament by painting females and controlled their behaviour by sedating them and moving them in a dance-like fashion by a motor. This experiment showed that males preferred ornamented females, even when female behaviour was standardized. Thus, ornament display accurately predicted the duration of female–female competition and mating success, and was used as a signal by males in their choice of mates.