Background Research has highlighted the need for improving the implementation of advance care planning (ACP) in nursing homes. We developed a theory-based multicomponent ACP intervention (the ACP+ programme) aimed at supporting nursing home staff with the implementation of ACP into routine nursing home care. We describe here the protocol of a cluster randomised controlled trial (RCT) that aims to evaluate the effects of ACP+ on nursing home staff and volunteer level outcomes and its underlying processes of change. Methods We will conduct a cluster RCT in Flanders, Belgium. Fourteen eligible nursing homes will be pair-matched and one from each pair will be randomised to either continue care and education as usual or to receive the ACP+ programme (a multicomponent programme which is delivered stepwise over an eight-month period with the help of an external trainer). Primary outcomes are: nursing home care staff's knowledge of, and self-efficacy regarding ACP. Secondary outcomes are: 1) nursing home care staff's attitudes towards ACP and ACP practices; 2) support staff's and volunteer's ACP practices and 3) support staff's and volunteers' self-efficacy. Measurements will be performed at baseline and eight months post-measurement, using structured self-reported questionnaires. A process evaluation will accompany the outcome evaluation in the intervention group, with measurements throughout and post-intervention to assess implementation, mechanisms of impact and context and will be carried out using a mixed-methods design. Discussion There is little high-quality evidence regarding the effectiveness and underlying processes of change of ACP in nursing homes. This combined outcome and process evaluation of the ACP+ programme aims to contribute to building the necessary evidence to improve ACP and its uptake for nursing home residents and their family.