The midlife is a time of opportunity for maintaining and improving one’s health. Health-promoting behaviours developed during the midlife have a large influence on health outcomes experienced in older adulthood. This study was therefore aimed at investigating the effectiveness of an Internet-delivered brief action planning goal-setting activity to increase physical activity levels among healthy middle-aged adults. At baseline, a total of 176 healthy middle-aged adults completed a set of online self-report questionnaires measuring psychosocial factors (i.e., control beliefs, emotion regulation, resilience, social support) as well as physical activity levels. A series of linear multiple regressions indicated that cognitive reappraisal significantly predicted walking, t(1,173) = 2.445, p = .015, support from friends significantly predicted moderate activities, t(1,173) = 2.104, p = .037, and resilience significantly predicted vigorous activities, t(1,173) = 2.139, p = .034. After completing the baseline questionnaires, participants were randomly assigned to an experimental group (i.e., brief action planning goal-setting activity) or a control group (i.e., goal-setting activity without brief action planning). At the 4-week follow-up period, 38 participants completed the same online self-report questionnaire measuring physical activity levels that was completed at baseline. A series of independent samples t-tests indicated that changes in physical activity levels from baseline to the 4-week follow-up period were not significantly different across the experimental and control groups. While brief action planning may not result in changes in physical activity levels, further investigation into the role of cognitive reappraisal, support from friends, and resilience among health middle-aged adults is warranted.