Theory of planned behaviour (TPB) studies have identified perceived behavioural control (PBC) as the key determinant of walking intentions. The present study investigated whether an intervention designed to alter PBC and create walking plans increased TPB measures concerning walking more, planning and objectively measured walking. One hundred and thirty UK adults participated in a waiting-list randomised controlled trial. The intervention consisted of strategies to boost PBC, plus volitional strategies to enact walking intentions. All TPB constructs were measured, along with self-reported measures of action planning and walking, and an objective pedometer measure of time spent walking. The intervention increased PBC, attitudes, intentions and objectively measured walking from 20 to 32 min a day. The effects of the intervention on intentions and behaviour were mediated by PBC, although the effects on PBC were not mediated by control beliefs. At 6 weeks follow-up, participants maintained their increases in walking. The findings of this study partially support the proposed causal nature of the extended TPB as a framework for developing and evaluating health behaviour change interventions. This is the first study using the TPB to develop, design and evaluate the components of an intervention which increased objectively measured behaviour, with effects mediated by TPB variables.