The Maui's Dolphin Challenge was a litter reduction project that was run twice at a secondary school in Aotearoa New Zealand. The project drew on a theoretical framework encompassing four psycho-social principles: values, embodied learning, efficacy, and perceived social norms. It challenged students to reduce the litter at the school by offering to donate $200 a week for 3 weeks to help protect the endangered Maui's dolphin. However, for every piece of litter found, $1 would come off this total. The challenge was accompanied by feedback on progress, posters, assemblies, and videos. Both times it was run, litter dropped by approximately half. After the first iteration, a survey found that students (n = 275 surveys) appeared motivated not to litter primarily due to a desire to care for the Maui's dolphin. Interviews and a focus group with staff and students (n = 14) after the second iteration also found the dolphins were important, but there was a cultural norm against picking up litter. The limitations of the project, its impact on the school's sustainability culture, and implications for other whole-school environmental projects, especially those with a waste focus, are discussed.