Aim: The academic performance of children impacts future educational attainment which may increase socioeconomic status which again influences their health. One of several factors that might affect academic performance is the diet. The aim of this study was to investigate the cross sectional relation between diet and self-reported reading-, writing-, and mathematical difficulties in Norwegian adolescents. Methods: In total, 475 ninth- and tenth-grade students out of 625 eligible ones from four different secondary schools in three different municipalities in Vest-Agder County, Norway, participated, giving a participation rate of 77%. The students filled in a questionnaire with food frequency questions of selected healthy and unhealthy food items, questions of meal frequency and different learning difficulties. Results: Regular breakfast was significantly associated with decreased odds of both writing and reading difficulties (OR: 0.44 (0.2–0.8), p = 0.01) and mathematical difficulties (OR: 0.33 (0.2–0.6), p ≤ 0.001). In addition, having lunch, dinner and supper regularly were associated with decreased odds of mathematical difficulties. Further, a high intake of foods representing a poor diet (sugar-sweetened soft drinks, sweets, chocolate, savory snacks, pizza and hot dogs) was significantly associated with increased odds of mathematical difficulties. Conclusions: Having a less-frequent intake of unhealthy foods and not skipping meals are associated with decreased odds of self-reported learning difficulties in Norwegian adolescents in this study. The results of this study support the need for a larger study with a more representative sample.