Background. Physical inactivity among Canadian youth is a growing concern. Socioeconomic status also appears to play a role, with youth from low-income families more likely to be inactive and/or overweight or obese. Step it Up is an after-school Boys and Girls’ Club program designed to increase physical activity (PA), improve nutrition, and help build developmental assets (e.g., social competency, positive values, etc.) among socially disadvantaged youth. Objectives. This study examined the impact of this program on participants’ PA and sedentary behaviour and PA cognitions using data from Brantford, Ontario, Canada. Methods. Fifty-eight children aged 6 to 11 years participated in the 2012-2013 program. A standardized PA intervention program was utilized (i.e., CATCH; Perry et al., 1990). Data was obtained from 23 participants (Response rate = 40%; Mage = 8.60; SD = 1.64; 55% Boys). Participants completed the Physical Activity Questionnaire for Older Children (PAQ-C; Kowalski et al., 1997), the physical activity self-efficacy scale (PASES; Saunders et al., 1997), subjective norms and attitudes towards PA based on the Theory of Planned Behaviour (TPB; Ajzen, 1991), as well as time spent in common sedentary activities (i.e., TV, video games, computer use) at three time-points: baseline (October 2012), mid-year (March 2013), and program completion (June 2013). Results. Repeated-measures ANOVAs revealed increases in PA (PAQ-C; p = .000; ?2 = .96), self-efficacy (p = .000; ?2 = .39), and attitudes (p = .014; ?2 = .35) as well as decreased sedentary time (p = .000; ?2 = .73). Contrary to expectations, subjective norms decreased (p = .000; ?2 = .84). Conclusions. The findings suggest the Step it Up Program was successful at increasing PA behaviour, self-efficacy, and attitudes and at decreasing sedentary time. The decrease in subjective norm suggests the need for PA interventions to extend beyond individual children by including their family and friends.