The purpose of this study was to assess the dietary glycemic index (GI) and glycemic load (GL) of adolescents, based on a Web-based 24-h recall, and to investigate dietary predictors of GI and GL. In addition, the relationship between GI and GL and weight status was examined. A Web-based 24-h recall was completed by 4936 adolescents, aged 9–17 years; macronutrient and food group intakes were assessed using the ESHA Food Processor, the Canadian Nutrient File, and Canada’s Food Guide. Dietary GI and GL were calculated based on published GI values for foods. Students provided self-reported height and mass. Multiple regression models assessed the ability of food group choices and food behaviours to predict GI and GL. Mean GI was 55 for girls and 56 for boys. Mean GL was 128 for girls and 168 for boys. Food group choices explained 26% of the variation in GI (p < 0.01) and 84% of the variation in GL (p < 0.01). The number of meals per day explained 10% (p < 0.01), and eating meals outside of the home accounted for 2.5% (p < 0.01) of the variation in GL; however, these results disappeared when adjusted for total energy intake. The GI was positively correlated with body mass index in girls (r = 0.05, p = 0.02), and GL was significantly higher among nonoverweight boys than overweight boys. This study identified eating patterns related to daily GI and GL, and suggests certain dietary patterns that could have beneficial effects on health. It also showed that GI and GL were weakly related to weight status.