Physical activity (PA) interventions in people with spinal cord injury (SCI) have been limited by a shortage of (1) evidence for sufficient increases in unsupervised PA to improve aerobic fitness and (2) stakeholder involvement in intervention design. This study examined the effects of a theory-based PA intervention, developed in collaboration with nearly 300 stakeholders, on PA levels, aerobic fitness, and psychosocial predictors of PA among individuals with SCI. A randomized controlled trial (RCT) was conducted with 28 men and women with chronic SCI (age 45.0 ± 11.5 years, years post-injury 16.4 ± 12.4). Participants randomized to the intervention group (n = 14) received an introductory personal training session followed by eight weekly 15-min PA behavioral coaching sessions per week. PA was assessed using self-report and accelerometers. Aerobic fitness and psychosocial predictors of exercise were evaluated using an incremental exercise test and survey methods, respectively. At post-intervention, controlling for baseline, the intervention group showed fivefold greater self-reported moderate to vigorous physical activity mean difference 247.9 min/day; 95% confidence interval (CI) 92.8-403.1; p = 0.026, d = 1.04, 17% greater accelerometer-measured PA (mean difference 3.9 × 10 vector magnitude counts; 95% CI 1.1 × 10 -7.7 × 10 ; p = 0.014, d = 0.31), and 19% higher peak oxygen uptake (VO ; mean difference 0.23 L/min; 95% CI 0.12-0.33; p < 0.001, d = 0.54) compared with the control group. Mean values of psychosocial predictors of PA were also significantly improved in the intervention group compared with controls. To our knowledge, this co-created behavioral intervention produced the largest effect size to date for change in self-reported PA in an RCT involving people with physical disability. This is also the first RCT in people with SCI to demonstrate that a behavioral intervention can sufficiently increase unsupervised PA to improve aerobic fitness. ClinicalTrials.gov, NCT03111030, 12 April 2017, https://clinicaltrials.gov/ct2/show/NCT03111030?term=NCT03111030&rank=1 .