Background: Interventions to increase physical activity or reduce sedentary behaviour within the workplace setting have shown mixed effects. This cluster randomised controlled trial assessed whether multi-component interventions, focusing on changes at the individual, environmental, and organisational levels, either increased physical activity or reduced sedentary behaviour, compared to a passive control group.Methods: Teams of office-workers from two companies participated in two interventions (iPA: targeting physical activity; iSED: targeting sedentary behaviour), or a waiting list control group. Exclusion criteria was a very high physical activity level. Randomisation was done on a cluster level, and groups were randomly allocated (1:1) with stratification for company and cluster size Personnel involved in data collection and processing were blinded for group allocation. Both interventions included five sessions of cognitive behavioural therapy counselling for 6-months. iPA included counselling focused on physical activity, access to a gym, and encouragement to exercise and go for lunch walks. iSED included counselling on sedentary behaviour and encouragement to reduce sitting and increase engagement in standing- and walking-meetings. At baseline and the 6-month mark accelerometers were worn on the hip and thigh for 7 days. The primary outcomes were group differences in time spent in moderate-to-vigorous physical activity (%MVPA) and in sedentary behaviour (%), analysed for those with complete data using Bayesian multilevel modelling.Results: 263 office-workers (73% women, mean age 42±9 years, education 15±2 years) in 23 cluster teams were randomised (iPA n=84, 8 clusters; iSED n=87, 7 clusters; C n=92, 7 clusters). No significant group differences (posterior mean ratios: 95% credible interval) were found after the intervention for %MVPA or for %Sedentary. %MVPA: iPA vs C (0·04: -0·80 – 0·82); iSED vs C (0·47: -0·41 – 1·32); iPA vs iSED (0·43: -0·42 – 1·27). %Sedentary: iPA vs C (1·16: -1·66 – 4·02); iSED vs C (-0·44: -3·50 – 2·64); iPA vs iSED (-1·60: -4·72 – 1·47).Conclusions: The multi-component interventions focusing on either physical activity or sedentary behaviour were unsuccessful at increasing device-measured physical activity or reducing sedentary behaviour compared to control.Trial registration: ISRCTN92968402