Purpose: The consumption of dietary flavonoids from plant-based foods has been related to the prevention of multiple chronic diseases. However, intake data from adolescents are lacking. We aimed to characterise the intake and major sources of dietary flavonoids among Australian adolescents and investigate changes during adolescence. Methods: The Raine Study Gen 2 participants completed a 212-item food frequency questionnaire at age 14 years and 17 years, with repeated measures for n = 883. Items were assigned a content for six flavonoid subclasses using the Phenol-Explorer database, which were summed for total flavonoid intake. Daily intakes and sources of flavonoids and flavonoid-subclasses were determined, and change assessed between 14 and 17 years, for males and females. Results: Major food sources of flavonoids and each subclass were similar at 14 and 17 years, with fruit juice the major contributor to total flavonoid intake at both time points (providing 44% and 38%, respectively). Citrus flavanones (predominantly hesperitin) were the major subclass at 14 years, while tea flavan-3-ols were a major subclass (predominantly procyanidin dimers) at 17 years. The mean intake of total flavonoids at 14 years was 210 ± 133 mg/day, reducing by 5% (10 mg/day) by 17 years. Females consumed a more flavonoid-dense diet compared to males (104.5 ± 71.5 mg/1000 kcal vs 80.4 ± 50.3 mg/1000 kcal per day; p < 0.001). Conclusion: This study provides a comprehensive estimation of flavonoid intake and their major food sources in a sample of Australian adolescents, which may be useful in the development of practical dietary recommendations.