Given the unpleasant nature of social isolation and loneliness (SIL) and their negative effects on health and wellbeing, interventions are needed. However, persistent issues in the design, evaluation, and reporting of SIL interventions preclude conclusive evidence and commentary on the effectiveness of SIL interventions. Here, we propose guidelines for evaluating SIL interventions, firstly by operationalising them into two categories: (1) interventions aiming to reduce SIL as a primary outcome and (2) interventions aiming to improve non-SIL outcomes in the lives of individuals experiencing SIL. Secondly, we evaluate instruments for measuring SIL and research designs for studying intervention effectiveness. Thirdly, guidelines for reporting information about the intervention, study design, results, and discussion in SIL intervention studies are presented. These guidelines will help researchers to better and more consistently report on SIL interventions and improve comparability of SIL interventions, ultimately contributing to the improvement of interventions and to the mitigation of SIL.