Background: Emerging evidence suggests that sedentary behaviour may be adversely associated with physical health, but few studies have examined the association with mental well-being. Purpose: This study examined the association of four non-occupational sedentary behaviours, individually and in total, with mental well-being in employed adults. Methods:Baseline data from the evaluation of Well@Work, a national workplace health promotion project conducted in the UK, were used. Participants self-reported sitting time whilst watching television, using a computer, socialising and travelling by motorised transport.Mental well-being was assessed by the 12-item version of the General Health Questionnaire (GHQ-12). Analyses were conducted using multiple linear regression. Results: In models adjusted for multiple confounders, TV viewing, computer use, and total non-occupational sitting time were adversely associated with GHQ-12 assessed mental well-being in women.No statistically significant associations were observed in men. Conclusions: Nonoccupational sitting is adversely associated with mental well-being in women but not men.