Background: The increasing consumption of fast food and junk food among adolescents is a growing public health concern, particularly in urban areas of developing countries like Bangladesh. This study aims to explore the relationship between knowledge of fast food and junk food consumption and sociodemographic characteristics among high school students in Bogura Town, Bangladesh. Methods: This cross-sectional descriptive study was conducted at the Department of Community Medicine, Rajshahi Medical College, from January 2023 to December 2023. A total of 384 high school students from various schools in Bogura were selected using purposive sampling. Data were collected using a pretested semi-structured questionnaire and analyzed using SPSS version 26. Descriptive statistics and chi-square tests were employed to assess associations between variables. Result: The majority of participants were aged 14-16 years (58.07%) and female (66.15%). Most students resided in urban areas (95.31%) and came from nuclear families (87.24%). Parental education levels were high, with 56.77% of fathers and 37.76% of mothers being graduates or having higher education. Despite this, only 23.70% of students had complete knowledge about the nutritional value of fast food and junk food. Significant associations were found between knowledge levels and gender, parental education, economic status, and BMI (p < 0.05). Fast food consumption was prevalent, with 74.48% consuming it at school and 78.13% consuming instant foods at home. Knowledge gaps were notable in areas such as the effects of saturated fats, metabolism, and chronic disease risks. Conclusion: The findings highlight significant gaps in nutritional knowledge among adolescents, despite high rates of fast food consumption. Sociodemographic factors play a crucial role in influencing nutritional knowledge. There is an urgent need for targeted educational programs to address these gaps and promote healthier eating habits among adolescents to improve long-term health outcomes.