Reducing morbidity and mortality associated with being overweight is a crucial public health goal. The aim of the present research was to test the efficacy of a very brief psychological intervention (a volitional help sheet) that could be used as an adjunct to standard weight loss programs to support increased weight loss in an overweight sample. Seventy-two overweight participants currently participating in a weight loss program were randomly allocated to either an intervention (volitional help sheet) condition or a control (distracter task) condition. The main outcome measure was weight at 1-month follow-up. Participants in both conditions lost significant amounts of weight, but those in the intervention condition lost significantly more than those in the control condition (d=0.66). The findings support the efficacy of the volitional help sheet to promote additional weight loss in an overweight sample engaged in a weight loss program. The volitional help sheet therefore represents a very brief, low-cost intervention that could be used to supplement ongoing weight-loss programs.