We propose a highly-informed, well-designed randomized controlled trial (RCT) that is critical for providing Class I evidence regarding an Internet-delivered physical activity (PA) intervention as a behavioral approach for managing slowed cognitive processing speed (CPS; the most common and perhaps most burdensome MS-related cognitive impairment) and its second learning and memory, symptomatic, and quality of life (QOL) correlates among fully-ambulatory persons with multiple sclerosis (MS) who present with CPS impairment. Such an approach will involve a single-blind, RCT that examines the effects of a remotely-delivered, Internet-based PA intervention compared with an active control condition for yielding immediate and sustained improvements in CPS, learning and memory, symptomatic, and QOL outcomes among persons with mild MS-related ambulatory impairment who demonstrate impaired CPS. The primary outcome is the raw (unadjusted), oral Symbol Digit Modalities Test (SDMT) score as a neuropsychological measure of CPS, and this will be collected remotely via screen-sharing technology. The secondary outcomes include an objective neuropsychological measure of learning and memory (California Verbal Learning Test-II) collected remotely via screen-sharing technology, self-report measures of fatigue (Fatigue Severity Scale), depressive symptoms and anxiety (Hospital Anxiety and Depression Scale), pain (Short-Form, McGill Pain Questionnaire) and QOL (Multiple Sclerosis Impact Scale-29) that will be captured remotely using Qualtrics. The tertiary outcome is accelerometry as an objective, device-based measure of steps/day that will be delivered and returned via pre-paid, pre-addressed envelopes through the United States Postal Service for generating a minimal clinically important difference value that guides the prescription of free-living PA for managing CPS impairment in clinical practice.