Providing information about the sugar content of packaged foods on product labels is an important strategy to lower consumers’ sugar intake. This study assessed the effect of exposure to different sugar labels on consumers’ understanding of the sugar content of foods and their food choices. In the first phase, five focus groups were conducted with a convenience sample of Brazilian adults to explore their perceptions about food labelling in general and sugar labelling in particular. Based on the qualitative results, four sugar label formats were developed and subsequently tested in a five-arm study on 1,277 adults via a randomised controlled online survey. The formats were: (i) no sugar information—control, (ii) total and added sugar content displayed in the Nutrition Information Panel (NIP), (iii) a front-of-package (FoP) octagonal warning for “high-in-sugar” products, (iv) a FoP magnifying glass warning for “high-in-sugar” products, and (v) a “high-in-sugar” warning text embedded on the NIP.