Few case studies exist in the public health or design literatures showing how to create national scale messaging campaigns in low-income countries using design processes. In this paper, we describe how we used Behaviour Centred Design to develop Nyumba ni choo, the Tanzanian National Sanitation Campaign. The process involved multiple iterations of ideation and filtration by professional creatives, government staff, academics and sanitation specialists to create a branded mass communication campaign, which was refreshed annually. The campaign was based on the insight that Tanzania is modernizing rapidly, with people upgrading their homes, but leaving their outside toilets in a 'traditional' state. Built around the 'big idea' that a household is not fully modern without a good-quality, modern toilet, the campaign employed reality TV shows, live engagements and mass and digital media postings, all targeted at motivating both the government and general population to improve toilets. The campaign has made toilets a topic of national conversation and has led to a major uptick in the rate of toilet building. Efforts to improve public health-related behaviour can be enhanced by using systematic approaches that build on available evidence, understand behaviour in its common settings, employ psychological theory and engage creative expertise.