Introduction: High levels of sugar intake are associated with multiple maladaptive health outcomes in adult and younger populations. Identifying the psychological determinants of sugar intake in adolescents, and the processes involved, may help identify potentially modifiable targets and inform intervention development. We tested the predictions of an extended theoretical model based on the theory of planned behavior (TPB), which specified social cognition constructs, habit, and self-control as correlates of sugar intake in an adolescent sample. Methods: Adolescents aged 12 to 14 years (N = 88) recruited via a survey panel company and consenting to participate in the study completed online self-report measures of constructs from the TPB alongside measures of habit and self-control. One month later, participants completed a follow-up measure of free-sugar intake. Hypothesized effects of our proposed extended model were tested using partial least squares structural equation modeling. Results: We found statistically significant effects of attitude, subjective norm, and perceived behavioral control on sugar intake intentions. We also found significant effects of habit and self-control on sugar intake measured at follow-up, but no effect for intention. Perceived behavioral control moderated the intention–behavior relationship such that intention effects on behavior were larger when perceived behavioral control was high. However, self-control did not moderate the intention–behavior relationship. Conclusion: Results indicate that sugar intake in this sample was a function of habits and self-control, and the effect of sugar intake intentions was conditional on perceived behavioral control. Results contribute to an evidence base of determinants and associated processes that relate to sugar intake in adolescents and may signal potentially modifiable targets for intervention.