["Psycho-Oncology, Volume 35, Issue 5, May 2026. ", "\nABSTRACT\n\nBackground\nAs research on behavioral interventions for young adult cancer survivors (YACS) progresses, understanding heterogeneity in positive affect experienced with health behaviors is essential to enhancing intervention effectiveness and improving survivorship outcomes.\n\n\nMethods\nGroup‐based multi‐trajectory modeling was conducted on data from the 6‐month intervention phase of the IMproving Physical Activity after Cancer Treatment (IMPACT) study to uncover trajectories of positive affect related to physical activity (PA) (PA enjoyment, mood after PA) among intervention participants (n = 140). Differences in baseline predictors were assessed using ANOVA and χ2, and moderate‐to‐vigorous physical activity (MVPA) measures were analyzed using multi‐level linear mixed models.\n\n\nResults\nThree trajectories were identified: Low‐Affect (n = 57), Mid‐Affect (n = 58), and High‐Affect (n = 22). Participants in Mid‐Affect and High‐Affect reported higher levels of being partnered and incomes > $60,000, while those in Low‐Affect reported worse baseline psychosocial and health‐related quality of life factors (p < 0.05). By 6 months, MVPA goal adherence for Low‐Affect and Mid‐Affect groups declined to 53% (SE = 13%) and 65% (SE = 13%), respectively, whereas participants in High‐Affect achieved the intervention's target of 150 MVPA min/week and sustained nearly 100% goal adherence. Increases in MVPA min/week over 6‐months were largest among those in High‐Affect (+32.3 [SE = 11.5]) compared to those in Low‐Affect (+18.9 [SE = 7.6]) and Mid‐Affect (+26.1 [SE = 7.3]), with no between‐group differences (p > 0.05).\n\n\nConclusions\nThese findings highlight variation in positive affect trajectories, their baseline predictors, and how they relate to MVPA goal adherence. Interventions that foster positive affect related to PA and offer support based on positive affect trajectory may improve MVPA and survivorship outcomes for YACS.\n\n\nClinical Trial Registration\nClinicalTrials.gov ID: NCT03569605, https://clinicaltrials.gov/study/NCT03569605.\n\n"]