Background: This study aimed to clarify factors affecting changes in moderate to vigorous physical activity (MVPA) in patients 1 to 3 months after undergoing percutaneous coronary intervention (PCI). Methods: In this prospective cohort study, we enrolled patients aged <75 years who underwent PCI. MVPA was objectively measured using an accelerometer at 1 and 3 months after hospital discharge. Factors associated with increased MVPA (≥150 min/wk at 3 mo) were analyzed in participants with MVPA < 150 minutes per week at 1 month. Univariate and multivariate logistic regression analyses were performed to explore variables potentially associated with increasing MVPA, using MVPA ≥ 150 minutes per week at 3 months as the dependent variable. Factors associated with decreased MVPA (<150 min/wk at 3 mo) were also analyzed in participants with MVPA ≥ 150 minutes per week at 1 month. Logistic regression analysis was performed to explore factors of declining MVPA, using MVPA < 150 minutes per week at 3 months as the dependent variable. Results: We analyzed 577 patients (median age 64 y, 13.5% female, and 20.6% acute coronary syndrome). Increased MVPA was significantly associated with participation in outpatient cardiac rehabilitation (odds ratio 3.67; 95% confidence interval, 1.22–11.0), left main trunk stenosis (13.0; 2.49–68.2), diabetes mellitus (0.42; 0.22–0.81), and hemoglobin (1.47, per 1 SD; 1.09–1.97). Decreased MVPA was significantly associated with depression (0.31; 0.14–0.74) and Self-Efficacy for Walking (0.92, per 1 point; 0.86–0.98). Conclusions: Identifying patient factors associated with changes in MVPA may provide insight into behavioral changes and help with individualized PA promotion.