OBJECTIVE: The aim of this preregistered, secondary analysis of a pilot randomized controlled trial (NCT02717429) was to compare the impact of 4-week mindfulness-based training and adaptive cognitive training, with a waitlist control condition, on processing speed and working memory in people with multiple sclerosis (PwMS). METHOD: Sixty-one PwMS were randomized to mindfulness-based training (MBT), adaptive computerized cognitive training (aCT), or a waitlist (WL) control group and completed the Brief Repeatable Battery of Neuropsychological Tests at pre- and posttraining. Training-related changes on the Symbol Digit Modalities Test (SDMT) and the Paced Auditory Serial Addition Test (PASAT) were the primary outcomes of interest. Baseline cognitive status was examined as a moderator of treatment gains. Practice time, change in aCT game difficulty, and rate of change in state awareness across MBT were assessed as correlates of cognitive gains. FINDINGS: Compared with aCT and WL, mindfulness training significantly improved processing speed (η CONCLUSIONS: In PwMS, 4 weeks of mindfulness meditation training improved processing speed above and beyond aCT and WL. More rapid change in awareness during mindfulness training may be associated with greater gains in working memory. (PsycInfo Database Record (c) 2020 APA, all rights reserved).