In France, 50% of consumers sweeten plain yogurts prior to consumption. This study measured how much sugar consumers added under contextualized testing conditions. Participants (199 French adults who regularly consume plain yogurt adding sugar) were given a plain yogurt (125 g) at the end of a full meal and were allowed to sweeten it with their usual sweetener (caster sugar, honey, or jam). The quantities added were measured indirectly by weighing the sweetener containers before and after use; they were then converted into equivalent quantities of sucrose, or “added sugar.” Participants were asked to describe their relative hunger, thirst, and liking for plain yogurt and to estimate the quantity of sweetener they had added. On average, participants added 13.6 g of sugar to their yogurts, which is higher than the 10.2 g of sugar contained in pre-sweetened commercial yogurts (125 g). More sugar was added when subjects used jam (24.4 g/yogurt, n = 36) as opposed to caster sugar (11.0 g/yogurt, n = 134) or honey (12.1 g/yogurt, n = 29). Age, socio-professional category, and BMI had a significant influence on added-sugar quantity. Based on behavior and attitude, participants could be separated into three evenly sized groups: “low sugar users” (n = 67, median = 6.1 g/yogurt), who tended to control their food intake, “medium sugar users” (n = 66, median = 11.4 g/yogurt), and “heavy sugar users” (n = 66, median = 19.9 g/yogurt) who sought immediate satisfaction. To our knowledge, this study is the first to provide robust data on the amount of sugar consumers add to plain yogurts in contextualized conditions (self preparation during a real meal). Our findings show that consumers underestimated by half the quantity of sweetener they added.