Objective: Mindfulness meditation reduces psychological distress among individuals with cancer. However, mechanisms for intervention effects have not been fully determined. This study tested emotion regulation strategies as mediators of intervention effects in a sample of younger women treated for breast cancer, a group at risk for psychological distress. We focused on two distinct strategies targeted by the intervention—rumination and self-kindness—and further examined the broader construct of mindfulness as a potential mediator. Method: Women (n = 71) with Stage 0-III breast cancer diagnosed at or before age 50 who had completed cancer treatment were randomly assigned to a 6-week mindfulness intervention or wait-list control group. Assessments occurred at study entry, postintervention, and a 3-month follow-up. Results: In single mediator analyses, increases in self-kindness (CIB [−7.83, −1.93]), decreases in rumination (CIB [−5.05, −.31]), and increases in mindfulness (CIB [−6.58, −.82]) each mediated reductions in depressive symptoms from pre- to postintervention. Increases in self-kindness also mediated reductions in perceived stress (CIB [−5.37, −.62]) from pre- to postintervention, and increases in self-kindness (CIB [−5.67, −.22]) and in mindfulness (CIB [−5.51, −.16]) each mediated intervention effects on perceived stress from preintervention to 3-month follow-up. In multiple mediator analysis, only self-kindness mediated intervention effects on depressive symptoms from pre- to postintervention (CIB [−6.41, −.61]), and self-kindness and mindfulness together mediated intervention effects on perceived stress from preintervention to follow-up (CIB [−6.77, −.35]). Conclusions: Self-kindness played a consistent role in reducing distress in younger women with breast cancer. The efficacy of this understudied emotion regulation strategy should be evaluated in other clinical populations.