Background:
Food parenting practices (FPPs) play a crucial role in shaping children's adulthood-portion size selection to avoid overeating (PSO), which are integral in the prevention of obesity, reducing overproduction, and reducing food waste. Aim: This study aimed to describe the relationship between several FPPs and implementation of PSO in adulthood and the mediating role of interest in health (IH) in adulthood and attitude toward food waste (AFW) in the above-mentioned association among Japanese adults. Methods: A total of 1707 Japanese adults who responded to an anonymous web-based questionnaire survey in February 2023 were included in the analysis (women: n = 865, 50.7%). Data on FPPs (nine items), PSO (eight items), IH (five items), AFW (three items), and demographic and anthropometric data were collected. Nine mediation models for each FPP were created, and mediation analysis with percentile-corrected bootstrap confidence intervals adjusted for demographic and anthropometric data was performed to examine the direct and mediation effects. Results: Significant direct effects were observed for all nine models (B = 0.33–0.74, p < 0.001). In addition, partial mediation was established in all the models without AFW for one FPP. Adulthood IH (indirect effect-adjusted model: B = 0.06–0.13) and AFW (B = −0.03–0.06, except for one FPP model) mediated the association between FPPs and adult PSO. The indirect effects accounted for 11.4–22.3% (IH) and 3.6–5.6% (AFW) of the total effects, respectively. Conclusion: The results provide new insights for the establishment of appropriate FPPs to promote PSO. This study suggests the potential impact of FPPs on PSO in adulthood.