High fast-food consumption is a common public-health concern among adolescents, due to its link to a number of non-communicable diseases. Frequent consumption of fast food may also affect diets of individuals; however, research addressing this issue is lacking in Saudi Arabia. We aimed to investigate the association between fast-food consumption, dietary quality, and dietary intake of adolescents in Saudi Arabia. This is a cross-sectional study of 617 healthy adolescents aged 11–18 years, who were recruited randomly from 16 middle- and high-schools located in Jeddah and Madinah, Saudi Arabia. Sociodemographic data were collected from parents. Dietary data, including the frequency of fast-food consumption, dietary quality (assessed using the short-form food frequency questionnaire), and dietary intake (assessed using multiple 24 h diet recalls from a subsample), were collected from the adolescents. Approximately one-third of adolescents (28.5%) reported frequent fast-food consumption (>two times a week). Results showed that a higher proportion of male adolescents frequently consumed fast-food, compared with female adolescents (32.8% vs. 24.8%, p = 0.039). Adolescents with the highest monthly family-income (≥SAR 21,000 ) reported a significantly higher frequency of fast-food consumption compared with families with a lower monthly income (p = 0.009). Frequency of fast-food consumption predicted lower dietary-quality in adolescents (Beta (B) = −0.27 [95% confidence interval (CI): −0.35 to –0.18]) and higher carbohydrate and free-sugar intake (B = 6.93 [95% CI: 0.78 to 13.1], and B = 3.93 [95% CI: 1.48 to 6.38], respectively). In conclusion, nutrition-intervention programs aiming to limit fast-food consumption and enhance the dietary quality of adolescents in Saudi Arabia, are warranted.