Objectives <p>Trauma burden and resulting psychopathology disproportionately affect Black adults with limited socioeconomic resources. Mindfulness-Based Cognitive Therapy (MBCT) is a group-based intervention with promise for addressing trauma-related psychopathology that may be of value for Black adults with limited socioeconomic resources because it may be more approachable for individuals with mental health stigma or hesitancy to engage with trauma memories. Trait mindfulness and emotion dysregulation (ED) are proposed mechanisms of action in MBCT, yet how these psychological factors shift as a result of MBCT in post-traumatic stress disorder (PTSD) symptom treatment for Black adults remains untested.</p> Method <p>The goal of this secondary analysis study was to examine changes in trait mindfulness and ED from a pilot randomized controlled trial (RCT) of an 8-week MBCT intervention adapted for Black trauma-exposed adults who screened positive for PTSD and depression compared to waitlist control (WLC). Eighty participants (86.3% women) were randomized. Trait mindfulness was measured across facets with the Five Facet Mindfulness Questionnaire: observing, describing, acting with awareness, nonjudgment, and nonreacting. ED was measured using the Difficulties in Emotion Regulation Scale. Data were collected at baseline (T1), post-treatment (or 8&#xa0;weeks following baseline for WLC) (T2), and 1-month follow-up (T3). Latent growth curve (LGC) models were run to examine change in trait mindfulness and ED by group status from T1 to T3.</p> Results <p>Mindful observing and nonreactivity increased over time (slope) in the MBCT group versus WLC (<i>β</i>_<i>slope</i> = 0.41, <i>SE</i> = 0.15, <i>p</i> = 0.005 and <i>β</i>_<i>slope</i> = 0.28, <i>SE</i> = 0.10, <i>p</i> = 0.007, respectively). Group status did not predict change in ED (<i>p</i> = 0.93).</p> Conclusions <p>Findings show that individuals in the MBCT group showed greater improvements in mindful observing and nonreactivity compared to WLC. Future efficacy trials in trauma-exposed Black adults are warranted to disentangle mechanisms of action and determine potential roles of trait mindfulness or ED in MBCT treatment outcomes.</p> Preregistration <p>The randomized controlled trial (RCT) that the data for this study came from was preregistered on Clinicaltrials.gov (NCT03922581).</p>