Purpose”:” Occupational sedentary behaviour (SB) is a public health concern associated with negative physical and psychological health consequences (1, 2). Reducing SB in office workers can be challenging due to the need to complete desk-bound work. There is evidence to suggest sit-stand workstations can reduce SB without negative effects on productivity (3), but are costly for companies to implement The aim was to evaluate the effects of a mobile app-based intervention targeting reductions in occupational SB, delivered with and without sit-stand workstations, on employee mood and productivity. Methods This 8-week feasibility cluster randomised controlled trial recruited desk-based office workers (n=56), aged 18-65 years, from three worksites in Northern Ireland. Following baseline measures, worksites were randomised to one of three groups”:” mobile app; mobile app and sit-stand workstation; or control. The “Worktivity” app, developed using the Behaviour Change Wheel (4) encouraged office workers to reduce sitting by self-monitoring SB and setting ‘sit-less’ goals. The app also delivered ‘sit-less’ nudges, educational prompts and progress reports. Mood and work productivity were measured at baseline, four and eight weeks. Productivity was measured daily for five consecutive days using ecological momentary assessment via text-message/e-mail where participants responded to a question relating to work productivity. Mood was measured using the Brunel Mood Scale (BRUMS). Results The intervention will conclude in December 2017 and findings will be available for ISBNPA 2018. As this is a feasibility trial, analysis will be mainly descriptive. Investigations will be exploratory to provide estimates of key parameters and inform the design of a definitive trial. Conclusions This study represents the first investigation of the effects of an app intervention, designed to reduce occupational SB, on employee productivity and mood. Findings are expected to inform the development of a larger-scale m-health intervention to reduce SB in office workers.