BackgroundTargeted self-management programmes may improve health and increase physical activity (PA) in people with multimorbidity.AimTo investigate the impact of a structured, theoretically driven, self-management group education programme on habitual PA levels in people with multimorbidity.Design and settingIndividually randomised controlled trial with 12-month follow-up, involving nine primary care practices in Leicestershire, UK.MethodIn total, 353 adults with multimorbidity (age 67.8 years [±9 years], 161 male sex) were randomised to intervention (n= 180) or control (n= 173) groups. Intervention participants were invited to attend four group-based self-management sessions, centred primarily on increasing PA, and received motivational text-message support. The primary outcome measure was change in overall volume (time and intensity) of daily PA at 12 months, as measured by the GENEActiv wrist-worn accelerometer device.ResultsAt baseline, the total sample achieved 22 min of moderate–vigorous intensity PA per day (mean/participant). At 12 months, in the complete-case analysis, a reduction in daily mean PA volume was seen in the intervention group relative to control (−0.80 milligravity [mg]; 95% confidence interval [CI] = −1.57 to −0.03;P= 0.04). Reductions were also seen in the intervention group in time spent in moderate–vigorous PA (−3.86 min per day; 95% CI= −6.70 to −1.03;P= 0.008) and time spent at an intensity equivalent to a slow walk (−4.66 min per day; 95% CI = −8.82 to −0.51;P= 0.028). However, the per-protocol analysis (excluding participants who did not attend at least one education session) found no between-group differences in overall daily PA at 12 months (−0.65 mg; 95% CI = −1.46 to 0.15;P= 0.11).ConclusionThe self-management programme elicited a slight reduction in PA levels in people with multimorbidity. Future research should identify and target subgroups of those with multimorbidity in greatest need of PA promotion in order to maximise potential capacity for benefit, and also focus on refining the intervention in order to increase efficacy in increasing PA.