In species where females preferentially select the most colourful males, males may strategically invest in courtship and nuptial colour according to the presence of rivals. In this experimental study, we tested this in the three-spined stickleback (Gasterosteus aculeatus) in which mature males exhibit carotenoid-based red coloration to attract mates and defend their territories against male competitors. We challenged experimental males with either a red-ornamented dummy male or a non-ornamented dummy for five min per day in six-d experimental trial, which was repeated twice during the breeding season. We found that the males presented with a coloured rival exhibited more frequent courtship behaviours (i.e. fanning and gluing) to females than those presented with a non-coloured intruder during the second experimental trial. At the end of each trial, the experimental males also showed a significantly larger area of red coloration in the presence of a coloured intruder. Our findings suggest that male sticklebacks regulate mating effort according to the presence of competitive rivals by increasing their investment in costly signals when successful mating and territory defence is at risk.