Background: Understanding the relationship between the perceived body weight, body mass index (BMI), and weight management-related behaviors is critical to preventing obesity in young adults. Purpose: To explore the relationship between perceived body weight and weight management-related behaviors and to assess the mediating effects of BMI between perceived body weight and weight management-related behaviors among young adults. Methods: Using data (n = 4882) from the National Longitudinal Study of Adolescent to Adult Health, multiple linear regression and a series of mediation analyses were applied to analyze the data. Results: Both perceived body weight and BMI were significantly associated with breakfast consumption and sleep for overall participants (p < 0.001; p < 0.001). Perceived body weight and BMI were significantly associated with breakfast consumption among young female adults (p < 0.001). Perceived body weight and BMI were significantly associated with sleep among young male adults (p = 0.03). BMI is found to mediate the relationship between perceived body weight and breakfast consumption. Discussion: The study demonstrated both perceived body weight and BMI contribute to certain weight management-related behaviors. BMI could be the inflammatory mechanism of health behavior change. Translation to Health Education Practice: Development of tailored interventions or culturally appropriate weight management educational programs are needed for young adults. Findings of this study could have implications for public health education.