Objective To assess the impact of cognitive Dual Task (DT) during walking on the energy cost of walking (Cw) in individuals with subacute stroke. The secondary objective was to determine whether there is an association between the Cw and cortical activity of the prefrontal area. Methods Any individual with stroke localized in the temporal-parietal regions and able to walk without human assistance was included. Cw and prefrontal cortical activity, recorded by fNIRS, were measured during simple task walking activity and cognitive dual task during walking. Results Nineteen individuals with stroke (age = 67.7 ± 9.6 yrs) were included. The cognitive DT during walking resulted in an increase in Cw of 23.1%; 95%CI [13.1%; 34.5%]. The increase in Cw in cognitive DT was correlated with the Cw for the single task walking activity (r = 0.48, p textless 0.01) as well as the predominance of cortical activity of the prefrontal area in the contralesional hemisphere (r = −0.33, p textless 0.01). Conclusion There is an increase in Cw during the cognitive DT. This increase is even more significant, as the Cw of the single task walking activity is high, and the cortical activity of the prefrontal areas predominates in the contralesional hemisphere.