Objectives Untreated dental caries can compromise oral health-related quality of life (OHRQoL). However, it is not clear which specific daily activities related to OHRQoL are compromised by the presence of this problem. Therefore, the purpose of this study was to identify which specific daily activities are affected by the presence of untreated dental caries using a representative sample of Brazilian children. Methods A cross-sectional study using a representative sample of Brazilian children (12 years of age) was conducted. The presence of untreated dental caries was assessed using the “decayed – D” component of the DMFT (decayed, missing, and filled teeth) index. Clinical exams were conducted by trained and calibrated dentists. The daily activities were evaluated using the survey Oral Impacts on Daily Performance (OIDP), considering items and domains. Descriptive, bivariate, and multiple analyses were conducted (α = 0.05). Results A total of 7,247 children were included, and 39.9 percent had at least one tooth with untreated dental caries, with 33.5 percent experiencing an impact on at least one of the OIDP activities. This study identified a greater chance of the presence of untreated dental caries in children having an impact on the psychological domain (P = 0.006), as well as on the “difficulty eating” (P = 0.007), and “difficulty sleeping” (P = 0.023) items. Conclusion The presence of untreated dental caries among Brazilian children was associated with an impact on specific daily activities (difficulty eating and sleeping) and the psychological domain of OHRQoL.