Aims To review the diabetes literature in order to examine the effect of motivational interventions on treatment outcome as measured by changes in glycated haemoglobin. Methods Relevant databases were systematically searched for randomised controlled trials in which motivational interventions were examined in relation to treatment outcome in people with type 1 and type 2 diabetes mellitus. Results The 13 studies identified for review included 1223 participants diagnosed with type 1 diabetes and 1895 participants diagnosed with type 2 diabetes. The analysis showed a 0.17% (95% CI: -0.09, 0.43%) improvement in glycemic control in people who received a motivational intervention compared to a control group, however, the effect was not statistically significant. Conclusions The impact of motivational interventions in the management of blood glucose levels appears to be limited. However, due to the small number of studies and issues of heterogeneity caution in interpreting the present findings is advised. Moreover, the unique contribution of motivational interventions may be better assessed by outcomes such as behaviour change and other intermediate outcomes. Further research examining the delivery and focus of motivational interventions in helping people manage their diabetes is recommended. The clinical implications of the present findings are therefore uncertain pending further research.