A multiple-baseline across-participants design was used to examine the effects of a Motivational General-Mastery imagery intervention on the sport confidence of 4 high-leveljunior badminton players. Sport confidence data were collected once a week for 21 weeks prior to international and county matches. The imagery intervention consisted of six imagery sessions (two per week for 3 weeks) and was administered using a multiple-baseline design with interventions commencing at Weeks 5, 7, 9, and 11 for Participants 1, 2, 3, and 4, respectively. Results of visual inspection and Binomial tests suggested significant increases in sport confidence for Participants 1 and 2, a significant decrease in sport confidence for Participant 3, and a delayed increase in sport confidence for Participant 4. The results are discussed in terms of the implications of using mastery imagery and the usefulness of multiple-baseline designs for furthering imagery research.