Objectives: Self-compassion entails relating to one’s negative experiences with awareness, acceptance, and kindness, and it is associated with greater well-being. The Self-Compassion Scale (SCS) includes mindfulness, which is theorized as a necessary precursor to a self-compassionate response. The present study examined associations of the SCS and its subscales with mindfulness and decentering at baseline and in daily life to clarify the measure’s construct validity. We also tested whether self-compassion moderates the association between mindfulness and eudaimonic well-being in daily life during occasions of increased negative affect. Methods: The sample of 172 community adults completed the SCS at baseline and a 7-day ecological momentary assessment. The SCS’s construct validity was tested with multilevel correlations and regressions. We tested the interaction of momentary mindfulness, momentary negative affect, and dispositional self-compassion in predicting momentary well-being. Results: Results generally supported the construct validity of the SCS, but SCS mindfulness subscales were most closely associated with decentering scales in daily life. Higher dispositional self-compassion, higher momentary mindful awareness, and lower momentary negative affect predicted higher momentary eudaimonic well-being. However, self-compassion did not interact with momentary mindful awareness and negative affect. Conclusions: The SCS generally related to measures of mindfulness and decentering as expected, but further work should be done to clarify subscale construct validity. Self-compassion was predictive of higher momentary eudaimonic well-being in people’s daily lives, supportive of ecological validity, but trait levels of self-compassion did not affect the relationship between momentary mindfulness and eudaimonic well-being.