Sedentary behaviour and physical inactivity are major health concerns, but research comparing messages that promote either a reduction in sedentary behaviour or an increase in physical activity is limited, especially those that also examine changes in their related cognitions. The purpose of this study was to test the impact of a sedentary behaviour reduction message versus a physical activity promotion message on behaviour-specific cognitions (e.g., self-efficacy and intentions). Participants (N = 64) were randomly shown either a sedentary behaviour (n = 31) or physical activity message (n = 33), and then reported their intentions and task self-efficacy for each behaviour. Effect size calculations (Cohen's d and partial eta square) were conducted to examine changes in sedentary behaviour and physical activity intentions from pre- to post-message exposure and post-message differences on sedentary behaviour and physical activity task self-efficacy. Regarding intentions, participants who read either message increased their physical activity and sedentary behaviour intentions (Cohen's d = .54 and .66, respectively); however, this change occurred irrespective of the message they read (partial eta square = .01 and .03, respectively). No post-message differences were found between message groups on physical activity task self-efficacy (Cohen's d = .13) and sedentary behaviour task self-efficacy (Cohen's d = .08). We observed that both sedentary behaviour and physical activity messages can influence intentions related to sedentary behaviour and physical activity. Thus, cognitions related to sedentary behaviour and physical activity do not appear to change only in response to a behaviour-specific message. More research is needed to examine the most effective strategies to independently target behaviour-specific cognitions.