The Lulun Project incorporated a social marketing strategy that accompanied a randomized controlled trial (RCT) of a food-based intervention that introduced eggs into the complementary feeding diet of Ecuadorian infants. This strategy was designed to promote behaviour change, in this case, egg consumption, through voluntary prosocial behaviour, empowerment, and brand loyalty. A three-phase social marketing strategy (design, campaigns, and evaluation) contributed to our successful RTC by applying techniques drawn from marketing, publicity, design, and communications. To develop the strategy, we conducted (a) market research focused on culturally based norms, values, and local expectations; (b) a situational assessment based on the four Ps of social marketing (people, product, place, and price); and (c) fostered a creative process to develop the project's brand and communication plan. The strategy combined a communication plan, brand, and activities that were implemented in four campaigns: outreach, recruitment, promotion, and closing. Our evaluation showed that the social marketing strategy was instrumental in promoting the RCT's objectives and responding to unforeseen events and community concerns regarding the RCT. The strategy resulted in high compliance, low attrition, and infant feeding policy change, including Ecuador's Ministry of Public Health new complementary feeding guidelines for introducing eggs early in complementary feeding. Use of social marketing techniques, like those in our study, could be key for scaling up this food-based intervention-or others like it-in Ecuador and beyond.