Background/Objectives: Physical activity (PA) is a key determinant of mental and physical health, yet its relationship with affective states remains insufficiently explored. Emotional factors, such as depression, anxiety, and motivation levels, can significantly impact PA engagement. This study aims to validate the Affective State and Physical Activity Questionnaire (ASPAQ), a novel 15-item instrument designed to assess the interplay between affective states and PA in adults. Methods: A cross-sectional study was conducted with 412 adults (145 males, 267 females, aged 18–65 years). Participants completed the ASPAQ alongside the International Physical Activity Questionnaire—Short Form (IPAQ-SF) and the Patient Health Questionnaire-9 (PHQ-9) on an online platform, with the support of trained operators. The psychometric properties of the ASPAQ were evaluated using reliability tests (Cronbach’s alpha, McDonald’s omega), exploratory factor analysis (EFA), and correlational analyses to assess convergent validity. Results: The ASPAQ demonstrated excellent reliability (Cronbach’s alpha = 0.973; McDonald’s omega = 0.973) and a unidimensional structure. Convergent validity was supported by significant correlations between ASPAQ scores and established measures of PA (IPAQ-SF) and depression (PHQ-9). EFA confirmed a single-factor model, reinforcing its conceptual integrity. Conclusions: The ASPAQ is a reliable and valid instrument for assessing the relationship between affective states and PA. Its integration with established measures offers a comprehensive tool for evaluating emotional barriers to PA. Future studies should explore its predictive validity and potential applications in clinical and public health settings to inform personalized interventions promoting PA among individuals with affective challenges.