BACKGROUND: Post-stroke physical inactivity is explained by several factors related to the stroke, which have been suggested as the causes and consequences of functional declines and health problems. Therefore, it is important to increase physical activity levels and reduce the time spent in low-energy expenditure activities after a stroke. Since the maintenance of cardiorespiratory fitness is a significant predictor of physical activity levels post-stroke, it may be important to investigate whether aerobic training is effective in increasing physical activity levels and reducing the time spent in low-energy expenditure activities in this population. The efficacy of aerobic training on these variables is not well known. The primary objective of this trial will be to investigate the effects of aerobic treadmill training on physical activity levels and on time spent in low-energy expenditure activities in people with stroke. The secondary aim will be to evaluate the effects of the training on cardiorespiratory fitness, endurance, depression, mobility, quality of life, and participation. METHODS/DESIGN: A randomized controlled trial, with blinded assessments, will be performed in a community-based setting. Altogether, 22 adults with a diagnosis of stroke (>6 months) who are sedentary or insufficiently active will be included. Participants will be randomly assigned to either: (1) aerobic treadmill training (experimental group, at 60-80% of their heart rate reserve) or (2) walking outside (control group, below 40% of heart rate reserve). Both groups will attend 40-min training sessions, three times per week over 12 weeks, in groups of two to four participants, with a trained physiotherapist. Primary outcomes are physical activity levels and time spent in low-energy expenditure activities (Multi-sensor SenseWear Mini® and Human Activity Profile). Secondary outcomes are cardiorespiratory fitness (peak oxygen uptake VO and ventilatory threshold), endurance, depression, mobility, quality of life, and participation. The effects of the training will be analyzed from the collected data using intention to treat. Between-group differences will be measured by two-way repeated measures ANOVA, considering the baseline, post-training, and 4-week follow-up. DISCUSSION: The results of this trial will likely provide valuable new information on the effects of aerobic treadmill training on physical activity levels and on time spent in low-energy expenditure activities of individuals with stroke, through changes in cardiorespiratory fitness. TRIAL REGISTRATION: ClinicalTrials.gov, NCT02798237 . Registered on 13 June 2016.