This study was an economic evaluation conducted alongside a cluster randomised controlled trial with a follow-up of 12 months. The aim was to evaluate the cost-effectiveness and cost-benefit of the Stay@Work Participatory Ergonomics programme (PE) compared to a control group (no PE). In total, 37 departments (n=3047 workers) were randomised into either the intervention (PE) or control group (no PE). During a meeting, working groups followed the steps of PE, and composed and prioritized ergonomic measures aimed to prevent low back pain (LBP) and neck pain (NP). Working groups had to implement the ergonomic measures within three months in their department. Cost data included those directly related to LBP and NP. Cost-effectiveness analyses (CEA) and cost-benefit analyses (CBA) were performed. After 12 months, health care costs and costs of productivity losses were higher in the intervention group than in the control group (the mean total cost difference was $/euro$127; 95% CI $/euro$-164 - $/euro$418). From a societal perspective, the CEA showed that PE was not cost-effective compared to control for LBP and NP prevalence, work performance, and sick leave. The CBA from a company perspective showed a monetary loss of $/euro$78 per worker. The PE programme was neither cost-effective nor cost-beneficial on any of the effect measures.