Background: Sufficient physical activity (PA) is important to reduce the risk of men developing chronic diseases and to improve mental health. The effectiveness of PA programmes can vary, however, among men. Individual and socio-psychological characteristics may affect the level of men's PA before starting a behaviour change programme as well as changes in PA during the programme. Aims: The primary objective of this study was to examine the role of individual and socio-psychological characteristics in predicting men's (a) accelerometer-assessed PA (step count and moderate to vigorous PA [MVPA]) upon presentation to a behaviour change programme and (b) changes in PA (step count and MVPA) during a behaviour change programme. Methods: A total of 109 men (mean age = 45.5 years, SD = 7.8 years, mean body mass index = 34.5 kg/m[superscript 2], SD = 5.1 kg/m[superscript 2]) who participated in the "Australian Fans in Training" 3-month PA and dietary behaviour change programme in 2018 participated in this study. Before and after completing the 12-week programme, men completed assessments including individual (age, weight) and socio-psychological (psychological needs support, psychological needs satisfaction, positive affect and negative affect) characteristics. We used regression analysis to examine the relationship between these variables and PA. Results: At baseline, men's weight ([beta]= --0.36, p < 0.001) and positive affect ([beta] = 0.29, p < 0.01) were significant predictors of step count. At baseline, men's weight ([beta] = --0.21, p < 0.05) and negative affect ([beta]= 0.23, p < 0.05) were significant predictors of MVPA, though the overall model did not display statistical significance. The overall regression models did not significantly predict changes in either step count or MVPA pre- to post-programme. Conclusion: The influence of weight and positive and negative affect in predicting PA outcomes suggests that tailoring men's health behaviour change interventions to individual circumstances and needs could enhance their effectiveness for some men.