This cross-sectional study was designed to investigate whether diet quality and eating behaviors could mediate the association between sleep quality and body mass index (BMI) in young adults. For all participants (n = 925; aged 21.4 ± 2.5 years; 77.8% women) we evaluated: BMI, sleep quality, diet quality, and eating behavior dimensions (emotional eating, cognitive restraint, and uncontrolled eating). Linear regression models were used to test associations between exposure and outcome variables. Path analysis was conducted with all potential mediators and covariates entered at the same time. Results showed that emotional eating (β = 0.04 [95% CI: 0.03; 0.06]), cognitive restraint (β = 0.03 [95% CI: 0.01; 0.04]), uncontrolled eating (β = 0.02 [95% CI: 0.01; 0.04]) and diet quality (β = −0.14 [95% CI: 0.19;-0.08]) were significantly associated with sleep quality. Additionally, BMI was significantly associated with PSQI score (β = 0.09 [95% CI: 0.01; 0.17]), emotional eating (β = 0.89 [95% CI: 0.60; 1.18]), and cognitive restraint (β = 1.37 [95% CI: 1.02; 1.71]). After testing for mediation, results revealed that emotional eating and cognitive restraint evidenced a significant mediating effect on the association between sleep quality and BMI. Additionally, diet quality was significantly associated with emotional eating (β = −0.35 [95% CI: 0.56;-0.13]), cognitive restraint (β = 0.53 [95% CI: 0.27; 0.79]), and uncontrolled eating (β = −0.49 [95% CI: 0.74;-0.25]). In conclusion, young adults with poor sleep quality are more likely to deal with negative emotions with food, which, in turn, could be associated with higher cognitive restraint, becoming a vicious cycle that has a negative impact on body weight. Our results also emphasize the role of eating behaviors as determinants of diet quality, highlighting the importance of considering sleep quality and eating behaviors when designing obesity prevention strategies in this population.