The combination of low physical activity rates and increased cardiovascular deaths indicate the overwhelming need for behaviour change interventions that can effectively promote physical activity among sedentary women. This 11-week randomised controlled trial examined the effects of an implementation intentions intervention on sedentary womens walking behaviour. Seventy-five women (M age = 48.17) were randomly assigned to either a control group where they were required to self-monitor their daily pedometer-determined step count or to an experimental group where they were asked to form specific walking plans (i.e. implementation intentions) every 6 weeks and to self-monitor their daily pedometer-determined step count. Measures of exercise intentions, perceived behavioural control, scheduling and barrier self-efficacy were administered at baseline, week 6 and week 11. Analyses indicated higher step counts over the first 6 weeks for women in the experimental condition (p < 0.02). Furthermore, higher self-efficacy to schedule (p < 0.01) and overcome walking barriers (p < 0.03), as well as higher perceptions of behavioural control (p < 0.02) were found at week 11 for women in the experimental versus control condition. However, none of the control beliefs were found to mediate the effects of the intervention on the womens walking behaviour. Furthermore, the intervention did not have any effect on the strength of the goal intention-behaviour relationship. The findings suggest implementation intentions are an effective strategy for initiating leisure-time walking within sedentary women.