Validation and Application of an Online Self-Assessment Questionnaire for the Assessment of Perception of Functional Foods: A Cross-Sectional Psychometric Study in Adults

<b>Background/Objectives:</b> The increasing interest in functional foods has highlighted the need to better understand consumer perceptions and their influence on dietary behaviours. This study aimed to evaluate the psychometric properties of the Questionnaire for the Assessment of Perception of Functional Foods (QAPAF) and apply it to a Portuguese adult population to explore associations with sociodemographic and lifestyle factors. <b>Methods:</b> Participants were recruited through convenience sampling; the achieved sample was predominantly female and highly educated. The 17-item QAPAF was assessed through exploratory factor analysis (EFA), internal consistency (Cronbach's alpha), and test-retest reliability. Associations between QAPAF scores and participant characteristics were analysed using non-parametric tests. <b>Results:</b> EFA supported a four-factor structure, explaining 58.8% of total variance. Internal consistency was acceptable (α = 0.70), and test-retest analysis (<i>n</i> = 25) showed no significant score differences, indicating temporal stability. QAPAF scores were significantly higher among participants with higher education and among non-smokers and non-drinkers. No associations were found with sex, BMI, or income. Participants with correct understanding of functional foods were more likely to reject misconceptions and express trust in professional recommendations. <b>Conclusions:</b> The QAPAF is a valid and reliable tool for assessing functional food perceptions. Its application provides insights into consumer attitudes and may support the design of targeted food literacy interventions. Generalizability is limited by the convenience sampling and by the predominance of female and highly educated participants; external validation in more diverse samples and cultural contexts is warranted.