Physical inactivity is a key risk factor for non-communicable diseases, and there is a need for interventions to increase the adoption and maintenance of regular physical activity. Interventions based on Acceptance and Commitment (ACT) have shown promise for promoting a range of health behaviours, including physical activity. The aims of this review were to (1) determine the effectiveness of ACT interventions for physical activity; and (2) identify the ACT processes, behaviour change techniques (BCTs) and intervention characteristics associated with ACT interventions. Eight electronic databases were searched for ACT interventions that aimed to increase physical activity. Seven eligible studies were included in the systematic review, and ACT processes, Behaviour Change Techniques and other intervention components and characteristics of the included interventions were coded. Six studies were randomised controlled trials that were included in a random-effects meta-analysis, which indicated small-to-moderate effects on physical activity (SMD = 0.32, 95% CI (0.07, 0.57), p = 0.01). ACT interventions show promise for increasing physical activity, but very few of the 'active ingredients' of ACT interventions could be characterised as BCTs. Future development of ACT interventions for physical activity should attempt to describe and name the ACT processes targeted by the intervention, and the BCTs used to target those processes.