BACKGROUND: Women with co-occurring posttraumatic stress disorder (PTSD) and substance use disorder (SUD) experience systemic barriers that place them in danger of poorer treatment outcomes. Some mindfulness-based interventions (MBIs) have demonstrated efficacy in reducing PTSD and SUD symptoms. Mindfulness practice is a core component of MBIs, thought to elicit and maintain positive behavioral change; however, no research to our knowledge has assessed the role of mindfulness practice on sustained treatment gains among women with co-occurring PTSD-SUD. Such research is necessary to better inform MBIs for dually diagnosed women. METHODS: This secondary analysis assessed whether post-intervention formal and informal mindfulness practice predicted reductions in PTSD symptoms and substance craving 6 months following an 8-session mindfulness-based relapse prevention intervention for women diagnosed with co-occurring PTSD-SUD (N = 23). Data were derived from a pilot randomized controlled trial evaluating the feasibility and preliminary efficacy of a trauma-integrated mindfulness-based relapse prevention program for women with co-occurring PTSD-SUD. RESULTS: Greater duration of formal mindfulness practice (i.e., minutes per practice) predicted reduced total PTSD symptoms ([Formula: see text] = - .670, p < .00), trauma-related avoidance ([Formula: see text] = - .564, p = .01), arousal and reactivity ([Formula: see text] = - .530, p = .02), and negative cognitions and mood ([Formula: see text] = - .780, p < .01) six months following treatment. Informal practice did not predict any outcomes. CONCLUSIONS: This research highlights the potential role of formal mindfulness practice in sustaining reductions in PTSD symptoms over time among women with co-occurring PTSD-SUD. Further study of strategies to promote ongoing formal mindfulness practice in this population following a MBI are warranted. Trial registration The parent trial was registered with ClinicalTrials.gov (Identifier: NCT03505749).